Exposure method for overlaying one mask pattern on another

ABSTRACT

An exposure method in which mask patterns are overlaid on one another on a substrate, which is an object to be exposed, by using a first and second exposure apparatuses having respective exposure fields of different sizes. The exposure method includes the steps of: sequentially transferring a first mask pattern onto the substrate in the form of a first array in units of a shot area of a predetermined size by using the first exposure apparatus; detecting at least either one of a perpendicularity error of the first array from a design value and a mean value of rotation angles of the shot areas in the first array when a second mask pattern is to be sequentially transferred onto the substrate. in the form of a second array in units of a shot area different in size from the unit shot area of a predetermined size by using the second exposure apparatus; and rotating the second mask pattern and the substrate relative to each other through an angle corresponding to a result of the detection, and thereafter, sequentially transferring the second mask pattern onto the substrate.

This is a Division of application Ser. No. 09/415,500 filed Oct. 12,1999, which in turn is a Division of application Ser. No. 09/236,090,filed Jan. 25, 1999 (now U.S. Pat. No. 5,989,761), which is turn is aContinuation of application Ser. No. 08/654,419, filed May 28, 1996 nowabandoned. The entire disclosure of the prior applications is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure method for transferring amask pattern onto a photosensitive substrate during photolithographyprocesses in the manufacture of semiconductor devices, liquid crystaldisplay devices, imaging devices (e.g. CCD), thin-film magnetic heads,etc. More particularly, the present invention relates to an exposuremethod which is suitably applied to a process in which exposure issequentially carried out by the mix-and-match method with respect to twolayers, that is, a layer called “middle layer”, which requires no highresolution, such as an ion-implanted layer used in production of asemiconductor memory or the like, and a layer called “critical layer”,which requires high resolution.

2. Related Background Art

Exposure apparatuses, e.g. step-and-repeat reduction projection typeexposure apparatuses (steppers), are used in photolithography processesfor producing semiconductor devices, liquid crystal display devices,etc. Generally, a semiconductor device such as a VLSI is formed bystacking a multiplicity of pattern layers on a wafer while effectingalignment for each layer. Among the pattern layers, a layer that needsthe highest resolution is called “critical layer”, and a layer thatneeds no high resolution, e.g. an ion-implanted layer used in productionof a semiconductor memory or the like, is called “middle layer”. Inother words, the line width of a pattern which is exposed for the middlelayer is wider than the line width of a pattern exposed for the criticallayer.

There has been an increasing tendency for recent VLSI manufacturingfactories to carry out exposure operations for different layers by usingrespective exposure apparatuses in a process for producing a single typeof VLSI in order to increase the throughput (i.e. the number of wafersprocessed per unit time) in the production process. Under thesecircumstances, it has become common practice to carry out what is called“mix-and-match” exposure. In the mix-and-match exposure process,exposure for the critical layer is carried out by using a first stepperof high resolution which performs one-shot exposure with ademagnification ratio of 5:1, and exposure for the middle layer iscarried out by using a second stepper of intermediate resolution whichperforms one-shot exposure with a demagnification ratio of 2.5:1. Inthis case, the size of the exposure field of the second stepper is twiceas large as that of the first stepper in both lengthwise and breadthwisedirections, and the throughput of the second stepper in the exposureprocess is approximately four times that of the first stepper. This willbe explained below with reference to FIG. 35.

Assuming that, as shown in FIG. 35, exposure units on a wafer which areto be exposed by the first stepper are square shot areas SA₁₁, SA₁₂,SA₁₃, SA₁₄, . . . each surrounded by sides which are parallel to X- andY-axes perpendicularly intersecting each other, an exposure area whichis to be exposed by the second stepper is a shot area SB₁ which is solarge as to substantially contain the four shot areas SA₁₁ to SA₁₄. Whenexposure is to be carried out by the second stepper over the four shotareas SA₁₁, SA₁₂, SA₁₃ and SA₁₄ exposed by the first stepper, the secondstepper effects alignment of the shot area SB₁, which corresponds to theexposure field of the second stepper, on the basis of alignment marks(wafer marks) attached to the shot areas SA₁₁ to SA₁₄.

There is another conventional exposure method in which, for example, astep-and-scan type scanning exposure apparatus with a demagnificationratio of 4:1 is combined with either the above-described first or secondstepper. The step-and-scan exposure is a process in which a shot area ona wafer which is to be exposed is stepped to a scanning start position,and thereafter a reticle, which serves as a mask, and the wafer aresynchronously scanned with respect to a projection optical system,thereby sequentially transferring a pattern on the reticle onto the shotarea. The exposure field of the scanning exposure apparatus is equal,for example, in the width of the non-scanning direction to the exposurefield of the first stepper, but the exposure field width in the scanningdirection of the scanning exposure apparatus is 1.5 times that of thefirst stepper. It should be noted that there are various combinations ofdifferent exposure field sizes of a plurality of exposure apparatusesused in the mix-and-match exposure method in addition to theabove-described combinations.

Thus, the throughput of an exposure process can be increased by carryingout a mix-and-match exposure process using different exposureapparatuses in combination according to the resolution required for eachlayer on a wafer as described above. However, when exposure apparatuseshaving respective exposure fields of different sizes are used incombination, if a perpendicularity error remains in the array of shotareas (i.e. shot array) of the preceding layer, i.e. if the anglebetween the X- and Y-axes of the shot array deviates from 90°, a givenoverlay error arises. Such a perpendicularity error is due to the factthat the feed directions of the wafer stage driven by motors are notaccurately perpendicular to each other.

For example, assuming that in FIG. 35 the imaginary straight line 23Apassing through the centers of the shot areas SA₁₃ and SA₁₄ in the fourshot areas of the preceding layer is parallel to the X-axis, if theangle between the X- and Y-axes of the shot array deviates from 90° byan angle (perpendicularity error) W, the imaginary straight line 24passing through the centers of the shot areas SA₁₁ and SA₁₃ tilts by theperpendicularity error W [rad] relative to the Y-axis. In this case, ifexposure is carried out with the center of the subsequent shot area SB₁aligned with the center 25 of the four shot areas SA₁₁, SA₁₂, SA₁₃ andSA₁₄, which have the perpendicularity error W, a uniform overlay errorΔx arises in the direction X between the pattern in the shot area SB₁and the pattern in each of the shot areas SA₁₁ to SA₁₄ of the precedinglayer. Assuming that the length of each side of the shot area SA₁₁ is L,the overlay error Δx is approximately L·W/2.

In a case where each shot area of the preceding layer has a shotrotation (chip rotation) also, an overlay error arises which is similarto that in a case where the angle between the X- and Y-axes of the shotarray deviates from 90°.

FIG. 36 shows the four shot areas SA₁₁ to SA₁₄ in a situation where theperpendicularity error of the shot array is zero, but the shot rotationis θ [rad]. Let us assume that the shot rotation θ is of the same sizeas the perpendicularity error W in FIG. 35. In the case of FIG. 36, evenif the subsequent shot area SB₁ is exposed by rotating it simply throughan angle corresponding to the shot rotation θ, a uniform overlay errorΔx of the same size as that in the case of FIG. 35 arises in thedirection of the shot rotation between the pattern in the shot area SB₁and the pattern in each of the shot areas SA₁₁ to SA₁₄ of the precedinglayer.

That is, when exposure is sequentially carried out by using exposureapparatuses having respective exposure fields of different sizes, if thearray of shot areas of the preceding layer has a perpendicularity erroror a shot rotation, a uniform overlay error arises if the subsequentshot areas are simply aligned with respect to the preceding shot areas.

On the other hand, in the above-described mix-and-match method, in whichafter a layer on a wafer has been exposed by a first exposure apparatus,overlay exposure is carried out on the preceding layer by using a secondexposure apparatus, the second exposure apparatus may effect alignmentby an enhanced global alignment (hereinafter referred to as “EGA”)method as disclosed, for example, in Japanese Patent ApplicationUnexamined Publication (KOKAI) (hereinafter referred to as “JP(A)”) No.61-44429 (corresponding to U.S. Pat. No. 4,780,617). In this case,however, some problems are experienced, which will be explained belowwith reference to FIGS. 37(a) to 38(c).

FIGS. 37(a), 37(b) and 37(c) illustrate a related art in which exposureis carried out by the mix-and-match method using two exposureapparatuses having respective exposure fields of the same size. First, apattern image of a reticle RA shown in FIG. 37(b) is transferred ontoeach of shot areas 129A, 129B, . . . , 129I of a first layer, which areshown by the chain lines in FIG. 37(a), on a wafer 20 by using a firstexposure apparatus. In this case, it is assumed that a coordinate systemthat defines each particular travel position of a wafer stage of thefirst exposure apparatus (i.e. stage coordinate system) comprises anX1-axis and a Y1-axis, and that the Y1-axis is tilted by an angle Wclockwise from an ideal Y1*-axis which is perpendicular to the X1-axis.Further, the reticle RA has two identical circuit patterns 112A and 112B(i.e. two-chip pattern) formed in a pattern area 42A. The rotation angleof the reticle RA has been set so that the circuit patterns 112A and112B are arrayed in a direction perpendicular to the X1-axis whenexposure is carried out.

As a result, the shot areas 129A to 129I of the first layer are arrayedat a predetermined pitch along each of the X1- and Y1-axes, and the shotarray has a perpendicularity error W. Further, two identical circuitpattern images are transferred onto each of the shot areas 129A to 129Iin such a manner as to lie in side-by-side relation to each other in adirection perpendicular to the X1-axis.

Next, a pattern image of a reticle RC shown in FIG. 37(c) is transferredonto each of shot areas of a second layer on the wafer 20 by using asecond exposure apparatus. In this case, it is assumed that a stagecoordinate system of the second exposure apparatus comprises an X2- axisand a Y2-axis, and that a direction corresponding to the X1-axis of thefirst layer on the wafer 20 has been set parallel to the X2-axis bypre-alignment carried out in the second exposure apparatus. Although theorigins of the coordinate systems (X1,Y1) and (X2,Y2) in FIG. 37(a) havebeen set at the center of the wafer 20 for the sake of explanation, itshould be noted that the origins of these coordinate systems may be setat any positions. The reticle RC also has two identical circuit patterns127A and 127B formed in a pattern area 42C, and the image of the patternarea 42A of the reticle RA as projected on the wafer 20 (i.e. exposurefield) and the projected image (exposure field) of the pattern area 42Cof the reticle RC are of the same size.

In this case, the second exposure apparatus effects alignment by theabove-described ECA method. That is, array coordinates of wafer marks(not shown) provided for a predetermined number of shot areas (sampleshots) selected from the first layer on the wafer 20 are measured tothereby calculate array coordinates of all the shot areas in the stagecoordinate system (X2,Y2). Thus, the second exposure apparatus canrecognize that the perpendicularity error W is present in the shot arrayon the first layer.

In the second exposure apparatus, therefore, the rotation angle of thereticle RC is set so that the two circuit patterns 127A and 127B arearrayed in a direction perpendicular to the X2-axis, as shown in FIG.37(c), and thereafter, a shot array of a second layer is set by takinginto consideration the perpendicularity error W. Then, exposure iscarried out. As a result, the circuit pattern images of the reticle RCare transferred onto each of shot areas 130A, 130B, . . . , 130I of thesecond layer, shown by the solid lines in FIG. 37(a), on the wafer 20.Thus, the shot array of the second layer is accurately overlaid on theshot array of the first layer.

In a case where the exposure fields (shot areas) of two exposureapparatuses have the same size as described above, even if the shotarray of the first layer has a perpendicularity error, the overlayaccuracy between the first and second layers can be maintained at highlevel by effecting alignment according to the EGA method, for example.

However, if the shot array of the first layer has a perpendicularityerror in a case where the exposure fields of the two exposureapparatuses have different sizes, the overlay accuracy between the twolayers cannot be increased above a certain level by an ordinary exposuremethod.

FIGS. 38(a), 38(b) and 38(c) illustrate a related art in which exposureis carried out by the mix-and-match method using two exposureapparatuses having respective exposure fields of different sizes. First,a pattern image of a two-chip reticle RA, which has two identicalpatterns 112A and 112B written in a pattern area 42A as shown in FIG.38(b), is transferred onto each of shot areas of a first layer on awafer 20 by using a first exposure apparatus. Next, a pattern image of athree-chip reticle RB, which has three identical circuit patterns 113Ato 113C written in a pattern area 42b as shown in FIG. 38(c), istransferred onto each of shot areas of a second layer on the wafer 20 bya second exposure apparatus. The image of the reticle RB as projected onthe wafer 20 has the same horizontal width as that of the projectedimage of the reticle RA, but the vertical width of the projected imageof the reticle RB is 3/2 times that of the reticle RA.

In this case also, the stage coordinate system of the firstexposure-apparatus is denoted by (X1,Y1), and the stage coordinatesystem of the second exposure apparatus is denoted by (X2,Y2), and it isassumed that alignment and exposure are carried out with the X2-axisaligned with the X1-axis. When exposure is carried out with the firstexposure apparatus by setting the reticle RA so that the two circuitpatterns of the reticle RA are arrayed in a direction perpendicular tothe X1-axis, the circuit patterns are transferred onto each of shotareas 129A, 129B, . . . , 129I of the first layer, shown by the chainlines in FIG. 38(a), on the wafer 20. In this case also, the array ofthe shot areas 129A to 129I has a perpendicularity error in the same wayas in the example shown in FIGS. 37(a) to 37(c).

Thereafter, the wafer 20 is aligned by the EGA method using a secondexposure apparatus, and then exposure is carried out in such a mannerthat the three circuit patterns of the reticle RB are arrayed in adirection perpendicular to the X2-axis. Consequently, the three circuitpatterns are transferred onto each of shot areas 131A to 131F of thesecond layer on the wafer 20, as shown by the solid lines in FIG. 38(a).However, because each shot area of the first layer has two circuitpatterns transferred thereto, while each shot area of the second layerhas three circuit patterns transferred thereto, the shot array of thefirst layer and the shot array of the second layer undesirably differfrom each other in the number of rows in a direction approximatelyperpendicular to the X1-axis. As a result, it becomes impossible toeliminate the effect of a perpendicularity error, which is an errorbetween the rows or columns of a shot array. For example, in FIG. 38(a),if the shot area 129A and the shot area 131A are aligned in thedirection X1 (or X2), a large overlay error arises in the direction X1between the shot area 129B and the shot area 131A.

Meanwhile, if both a first and second exposure apparatuses employ theEGA method, the following problems arise. The problems will be explainedbelow with reference to FIGS. 39(a) to 41(b).

In this EGA process, array coordinates of a predetermined number of shotareas (sample shots), which have previously been selected from amongshot areas on a wafer, are measured to determine, for example, sixcoordinate transformation parameters for calculating array coordinatesin a stage coordinate system, in which the wafer stage is to bepositioned, from the design array coordinates of all the shot areas.

However, when a pattern for a middle layer is transferred onto acritical layer by the mix-and-match exposure method, for example, agiven overlay error may remain if the coordinate transformationparameters obtained by the EGA method (hereinafter occasionally referredto as “EGA parameters”) are used as they are because differentprojection exposure apparatuses are used for the critical and middlelayers. This means that the EGA parameters may have residual errors. Inorder to correct such residual errors, the conventional practice is tomeasure overlay errors by conducting test printing using marks foroverlay accuracy measurement (hereinafter referred to as “verniermarks”), as described below.

FIG. 39(a) shows a wafer 20 having vernier marks formed by a projectionexposure apparatus for exposure of a critical layer. In FIG. 39(a), shotareas SE1, SE2, . . . , SEM (M is an integer of 12 or more, for example)are arrayed on the wafer 20 at a predetermined pitch along each of theX- and Y-axes of an orthogonal coordinate system (X,Y). In each shotarea SEm (m=1 to M), alignment marks (wafer marks) and overlay accuracymeasuring vernier marks have been formed.

FIG. 39(b) is an enlarged view showing the mark arrangement in a shotarea SEm. In FIG. 39(b), the shot area SEm has a wafer mark 221X for theX-axis formed at an end in the direction +Y. The wafer mark 221Xcomprises line-and-space patterns arranged at a predetermined pitch inthe direction X. The shot area SEm further has a wafer mark 221Y for theY-axis formed at an end in the direction +X. The wafer mark 221Ycomprises line-and-space patterns arranged at a predetermined pitch inthe direction Y. The wafer marks 221X and 221Y are marks which aredetected by an imaging detection method (FIA method). Further, the shotarea SEm has vernier marks 222A to 222E formed therein at respectivepositions which are distributed in a cross shape. The vernier marks 222Ato 222E are, for example, box-in-box marks which are detected by animaging detection method (image processing detection method).

Next, predetermined vernier marks are overlaid on the wafer 20 shown inFIG. 39(a) by exposure using a projection exposure apparatus for amiddle layer. For the overlay exposure, it is necessary to obtain arraycoordinates of each shot area SEm (m=1 to M) on the wafer 20 in thestage coordinate system of the projection exposure apparatus for amiddle layer. Therefore, it is assumed that the wafer marks 221A and221Y of each shot area SEm (m=1 to M) indicate the coordinates of thecenter of the corresponding shot area. It is further assumed that thedesign array coordinates of the center of each shot area SEm (m=1 to M)in the coordinate system on the wafer 20 (i.e. the sample coordinatesystem) are (Dxn,Dyn), and that the computational array coordinates ofeach shot area SEm (m=1 to M) in the stage coordinate system of theprojection exposure apparatus for a middle layer are (Fxn,Fyn). In thiscase, the X component Dxn and Y component Dyn of the design arraycoordinates of the center of each shot area SEm are the X coordinate ofthe corresponding wafer mark 221X and the Y coordinate of thecorresponding wafer mark 221Y, respectively, which may be approximatelyexpressed by the following equation (1): $\begin{matrix}{\begin{bmatrix}{Fxn} \\{Fyn}\end{bmatrix} = {{\begin{bmatrix}{Rx} & {- {{Rx}( {W + \theta} )}} \\{{Ry} \cdot \theta} & {Ry}\end{bmatrix}\begin{bmatrix}{Dxn} \\{Dyn}\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (1)\end{matrix}$

The transformation matrix in Eq. (1) has as elements six coordinatetransformation parameters (EGA parameters), including scaling parametersRx and Ry, rotation θ, perpendicularity W, and offsets Ox and Oy. Thescaling parameters Rx and Ry are linear expansion and contractionquantities in the directions X and Y, respectively. The rotation θ is anangle of rotation of the wafer 20. The perpendicularity W is aperpendicularity error, that is, a deviation of the intersection anglebetween the X- and Y-axes from 90°. The offsets Ox and Oy are shiftquantities in the directions X and Y, respectively. Next, in order todetermine values of the six coordinate transformation parameters, theprojection exposure apparatus for a middle layer measures arraycoordinates in the stage coordinate system of the wafer marks 221X and221Y provided for each of, for example, 10 shot areas (sample shots)SEa, SEb, SEc, . . . , SEj selected from among the shot areas on thewafer 20 shown in FIG. 39(a). The sample shots SEa to SEj are disposedat the vertices of an approximately regular polygon on the surface ofthe wafer 20 or at uniformly dispersed random positions.

In this case, the measured values of the array coordinates in the stagecoordinate system of the wafer marks 221X and 221Y obtained by the n-thmeasuring operation (n=1 to 10), that is, the measured array coordinatesof the center of the n-th sample shot, are assumed to be (Mxn,Myn).Next, the design array coordinates (Dxy,Dyn) of the wafer marks 221X and221Y are substituted into the right-hand side of Eq. (1) to obtaincomputational array coordinate values (Fxn,Fyn). Then, deviations of themeasured coordinate values (Mxn,Myn) from the computational arraycoordinate values (Fxn,Fyn), that is, alignment errors(Exn,Eyn)(=(Mxn−Fxn,Myn−Fyn)), are obtained. Thereafter, values of thesix EGA parameters are determined so as to minimize the sum of thesquares of the alignment errors obtained for all the sample shots, thatis, the residual error component.

Assuming that the number of measured sample shots is K (K=10 in FIG.39(a)), the residual error component is expressed by the followingequation (2). For example, values of the six EGA parameters (scalingparameters Rx, Ry, wafer rotation θ, perpendicularity W, and offsetquantities Ox, Oy) are obtained by solving simultaneous equationsestablished by setting the result of partial differentiation of Eq. 2with respect to each of the six EGA parameters equal to zero.$\begin{matrix}{{{Residual}\quad {error}\quad {component}} = {\sum\limits_{n = 1}^{K}\{ {( {{Mxn} - {FXn}} )^{2} + ( {{Myn} - {Fyn}} )^{2}} \}}} & (2)\end{matrix}$

Next, the six EGA parameter values thus obtained and the design arraycoordinate values (Dxm,Dym) of each shot area SEm (m=1 to M) aresequentially substituted into the right-hand side of Eq. (1), therebyobtaining array coordinate values in the stage coordinate system of eachshot area SEm of the critical layer on the wafer 20. Assuming that thedemagnification ratio for the critical layer is 5:1, while thedemagnification ratio for the middle layer is 2.5:1, that is, theexposure field of the projection exposure apparatus for the middle layeris 2 times as large as the-exposure field of the projection exposureapparatus for the critical layer in both the directions X and Y, eachmiddle layer shot area contains four critical layer shot areas.

Therefore, when exposure is to be carried out by the middle layerprojection exposure apparatus, the critical layer shot areas SEm (m=1 toM) shown in FIG. 39(a) are divided into a plurality of blocks eachcomprising two shot areas in the direction X and two shot areas in thedirection Y, and array coordinates in the stage coordinate system of thecenter of each block are obtained from the computational arraycoordinates of the four shot areas in the block. Thereafter, the arraycoordinates of the center of each block on the wafer 20 are sequentiallyaligned with the center of the exposure field of the middle layerprojection 15 exposure apparatus, and a pattern image of a reticle forthe middle layer, which contains vernier marks, is transferred onto eachblock by exposure. After the exposure process, the wafer 20 is subjectedto development process.

FIG. 40(a) shows the wafer 20 having overlaid vernier marks formed bythe middle layer projection exposure apparatus. In FIG. 40(a), shotareas SF1, SF2, . . . , SFN (N is an integer of 3 or more, for example)of the middle layer are arrayed on the wafer 20 at a predetermined pitchalong each of the X- and Y-axes, and each shot area SFn (n=1 to N)contains four critical layer shot areas. Further, the center 261 of eachshot area SFn is approximately coincident with the center of thecorresponding block of four critical layer shot areas. Each shot areaSFn has 20 (=4×5) vernier marks corresponding to a total of 20 verniermarks of the critical layer, that is, four groups of five vernier marks222A to 222E (see FIG. 40(b)).

Here, four shot areas SFa to SFd (shaded shot areas in FIG. 40(a)) aredefined as objects to be measured, and amounts of positionaldisplacement of the middle layer vernier marks relative to the criticallayer vernier marks are measured, for example, at measuring points 262to 265 selected at random in the shot areas SFa to SFd. FIG. 40(b) showsthe shot area SFa among the four. In FIG. 40(b), the middle layer shotarea SFa has middle layer vernier marks 224A to 224E, 226A to 226E, 228Ato 228E, and 230A to 230E formed to surround the vernier marks,respectively, which belong to four critical layer shot areas SEp,SE(p+1), SEq and SE(q+1), which underlie the shot area SFa. Accordingly,at the measuring point 262 in the shot area SFa, an amount of positionaldisplacement in the direction X or Y of the middle layer vernier mark226C relative to the critical layer vernier mark 222C in the shot areaSE(p+1) is measured. Similarly, an amount of positional displacementbetween the two corresponding vernier marks is measured at each of themeasuring points 263 to 265.

Consequently, if all the critical layer vernier marks are displaced, forexample, by a predetermined amount δX in the direction X relative to themiddle layer vernier marks at all the measuring points 262 to 265, inFIG. 40(a), it is revealed that the X-axis offset Ox in the EGAparameters has a residual error δX. Therefore, the residual error ispreviously stored in a control system of the middle layer projectionexposure apparatus as a system constant to correct an alignment result,thereby making it possible to form a middle layer pattern over thecritical layer by exposure with high overlay accuracy.

Thus, residual errors of the EGA parameters can be corrected bymeasuring amounts of positional displacement between the critical layervernier marks and the middle layer vernier marks. However, no particularconsideration has heretofore been given to the arrangement of measuringpoints for measuring amounts of positional displacement between thecritical layer vernier marks and the middle layer vernier marks, asshown by the measuring points 262 to 265 in FIG. 40(a). Accordingly,when the projected image for the middle layer has a magnification erroror a rotation error, for example, the magnification or rotation errormay be erroneously judged to be a residual error of the EGA parameters.

The above problem will be explained below with reference to FIGS. 40(a)to 41(b). FIG. 41(a) shows a state where the middle layer shot area SFais slightly enlarged relative to a projected image 266 obtained whenthere is no magnification error. As shown in FIG. 41(a), in the centralportion at the right end of the first quadrant of the shot area SFa(i.e. the critical layer shot area SE(p+1)), the middle layer verniermark 226C is displaced relative to the critical layer vernier mark 222Cby Δx1 and Δy1 in the directions X and Y, respectively. In the centerportion at the right end of the second quadrant (i.e. the shot areaSEp), the middle layer vernier mark 224C is displaced relative to thecritical layer vernier mark 222C by approximately Δy1 in the directionY, but the amount of displacement in the direction X of the middle layervernier mark 224C is so small as to be ignorable. Similarly, in thethird quadrant (i.e. the shot area SEq) and the fourth quadrant (i.e.the shot area SE(q+1)), the two vernier marks are displaced in symmetricrelation to those in the second and first quadrants, respectively.

When a projected image of the middle layer has such a magnificationerror, if an amount of positional displacement in the direction Xbetween the two corresponding vernier marks is measured at the measuringpoint 265 in the first quadrant of the shot area SFd, shown in FIG.40(a), and at the measuring point 263 in the second quadrant of the shotarea SFb, shown in FIG. 40(a), the results of the measurement are Δx1and 0, respectively. Accordingly, if residual errors of the EGAparameters of Eq. (1) are obtained by simply processing these amounts ofpositional displacement, predetermined errors remain in the scalingparameter Rx and offset Ox in the direction X, respectively.

If an amount of positional displacement in the direction X between thetwo corresponding vernier marks is measured at the measuring point 262in the first quadrant of the shot area SFa, shown in FIG. 40(a), and atthe measuring point 264 in the second quadrant of the shot area SFc,shown in FIG. 40(a), the results of the measurement are Δx1 and 0,respectively. Accordingly, if residual errors of the EGA parameters ofEq. (1) are obtained by simply processing these amounts of positionaldisplacement, predetermined errors remain in the perpendicularity W andthe offset Ox in the direction X, respectively. That is, if an amount ofpositional displacement in the direction X between two correspondingvernier marks is measured at measuring points in middle layer shot areasdefined as objects to be measured, which measuring points are indifferent columns on the critical layer, the magnification error of themiddle layer may be mistaken for a residual error (linear error) in theEGA parameters. Such erroneous recognition may also occur in the case ofmeasuring an amount of positional displacement in the direction Ybetween two corresponding vernier marks.

FIG. 41(b) shows a state where the middle layer shot area SFa has beenrotated counterclockwise relative to the projected image 266 obtainedwhen there is no error (i.e. a state where the shot area SFa has a shotrotation error). As shown in FIG. 41(b), in the central portion at theright end of the first quadrant of the shot area SFa, the middle layervernier mark 226C is displaced relative to the critical layer verniermark 222C by −Δx2 and Δy2 in the directions X and Y, respectively. Inthe central portion at the right end of the second quadrant (i.e. theshot area SEp), the middle layer vernier mark 224C is displaced relativeto the critical layer vernier mark 222C by approximately −Δx3 in thedirection X, but the amount of displacement in the direction Y of themiddle layer vernier mark 224C is so small as to be ignorable.Similarly, in the third and fourth quadrants, the two correspondingvernier marks are displaced in symmetric relation to those in the secondand first quadrants, respectively.

When a projected image of the middle layer has such a rotation error, ifan amount of positional displacement in the direction X between twocorresponding vernier marks is measured at the measuring point 265 inthe first quadrant of the shot area SFd, shown in FIG. 40(a), and at themeasuring point 263 in the second quadrant of the shot area SFb, shownin FIG. 40(a), the results of the measurement are −Δx2 and −Δx3,respectively. Accordingly, if residual errors in the EGA parameters ofEq. (1) are obtained by simply processing these amounts of positionaldisplacement, an error remains in a parameter other than the offset Oxamong the EGA parameters of Eq. (1). When an amount of positionaldisplacement in the direction Y between two corresponding vernier marksis measured at each of the measuring points 265 and 263, an errorsimilarly remains in an EGA parameter other than the offset Oy. Thus, itwill be understood that, when amounts of positional displacement betweenthe critical layer vernier marks and the middle layer vernier marks aremeasured to correct residual errors of the EGA parameters, a meremagnification error or rotation error of a middle layer shot area may bemistaken for a residual error of an EGA parameter other than the offsetsOx and Oy depending upon the selection of the positions of measuringpoints in middle layer shot areas as objects to be measured.

When critical layer shot areas (chip patterns) have a magnificationerror or a rotation error (chip rotation), such an error may also bemistaken for a residual error of an EGA parameter other than the offsetsOx and Oy depending upon the selection of measuring points for measuringamounts of positional displacement between the corresponding verniermarks.

As has been described above, residual errors of the EGA parameters canbe corrected by measuring amounts of positional displacement between thecritical layer vernier marks and the middle layer vernier marks.However, there may be residual errors not only in the above-describedcoordinate transformation parameters related to the whole wafer but alsoin so-called in-shot parameters comprising shot magnifications (i.e.linear expansion and contraction of each chip pattern in the directionsX and Y) rx and ry, shot rotation (i.e. a rotation angle of each chippattern) θ, and shot perpendicularity (i.e. a perpendicularity error ofthe coordinate system in each chip pattern) w.

To obtain a correction value for the shot magnification rx, for example,it is conceivable to measure an amount of positional displacementbetween two corresponding vernier marks at each of the two oppositemeasuring points 262 and 266 in the shot area SFa shown in FIG. 40(a). Aresidual shot magnification error, i.e. a correction value for the shotmagnification rx, should be calculable from the difference between the Xcomponents of the amounts of positional displacement measured at the twomeasuring points 262 and 266. Similarly, a residual shot rotation errorshould be calculable.

In actual practice, however, the vernier mark positions on the criticallayer may have different stepping errors because the measuring points262 and 266 belong to different shot areas SE(p+1) and SEp on thecritical layer. That is, if no consideration is given to the arrangementof measuring points at which vernier marks are to be read, variation dueto the stepping accuracy of the wafer stage may be mistaken for aresidual shot magnification error or a residual shot rotation error. Ifan erroneous correction value is used to correct the correspondingin-shot parameter, the alignment accuracy reduces disadvantageously.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide an exposure method capable of minimizing anoverlay error when a preceding layer has a perpendicularity error in anarray of shot areas or a shot rotation in a case where exposure iscarried out by the mix-and-match method using a plurality of exposureapparatuses having respective exposure fields of different sizes.

Another object of the present invention is to provide an exposure methodcapable of minimizing an overlay error when a perpendicularity errorremains in a shot array on a first layer in a case where exposure iscarried out by the mix-and-match method using a plurality of exposureapparatuses which are different from each other in the size of exposurefield (shot area) on a photosensitive substrate.

Still another object of the present invention is to provide an exposuremethod capable of increasing an overlay accuracy between a criticallayer pattern and a middle layer pattern in a case where exposure iscarried out by the mix-and-match method with respect to a substratewhere a critical layer and a middle layer are mixedly present.

The present invention provides an exposure method in which mask patternsare overlaid on one another on a substrate, which is an object to beexposed, by using a first and second exposure apparatuses havingrespective exposure fields of different sizes. The exposure methodincludes the steps of: sequentially transferring a first mask patternonto the substrate in the form of a first array in units of a shot areaof a predetermined size by using the first exposure apparatus; detectingat least either one of a perpendicularity error of the first array froma design value and a mean value of rotation angles of the shot areas inthe first array when a second mask pattern is to be sequentiallytransferred onto the substrate in the form of a second array in units ofa shot area different in size from the unit shot area of a predeterminedsize by using the second exposure apparatus; and rotating the secondmask pattern and the substrate relative to each other through an anglecorresponding to a result of the detection, and thereafter, sequentiallytransferring the second mask pattern onto the substrate.

The function of the above-described exposure method according to thepresent invention will be explained below. Let us assume that theperpendicularity of the first array, which is an array of shot areas towhich the first mask pattern is to be transferred, has an angle W ofdeviation from a design value (90° in general). Further, it is assumedthat each of these shot areas has a square outer shape, and that oneshot area in a second shot array to which a second mask pattern is to betransferred is laid over four shot areas in the first array. In thiscase, according to the present invention, the second mask pattern andthe substrate are rotated relative to each other so that the angle δ ofrotation of the shot area in the second array, which is defined aboutthe center of the four shot areas, is W/4. By doing so, the overlayerror between the first mask pattern image and the second mask patternimage on the substrate is reduced to a minimum on the average.

On the other hand, when the perpendicularity error of the first array iszero and the shot rotation of the four shot areas is W, the rotationangle δ of the shot area over the four shot areas is also set at W/4,whereby the overlay error between the first mask pattern image and thesecond mask pattern image on the substrate is reduced to a minimum onthe average.

In addition, the present invention provides another exposure method inwhich mask patterns are overlaid on one another on a substrate, which isan object to be exposed, by using a first exposure apparatus having afirst exposure field of a predetermined size, and a second exposureapparatus which scans a mask and the substrate synchronously tosequentially transfer a pattern formed on the mask onto the substrate,and which has a second exposure field different in size from the firstexposure field. The exposure method includes the steps of: sequentiallytransferring a first mask pattern onto the substrate in the form of afirst array in units of a shot area of a predetermined size by using thefirst exposure apparatus; detecting at least either one of aperpendicularity error of the first array from a design value and a meanvalue of rotation angles of the shot areas in the first array when asecond mask pattern is to be sequentially transferred onto the substratein the form of a second array over the first array in units of a shotarea different in size from the unit shot area of a predetermined sizeby using the second exposure apparatus; and displacing the second maskpattern and the substrate relative to each other in a directionperpendicular to a scanning direction of the second exposure apparatusby a distance corresponding to a result of the detection, andthereafter, sequentially transferring the second mask pattern ontothe-substrate by a scanning exposure method.

In this case, it is desirable to rotate the second mask pattern and thesubstrate relative to each other through an angle corresponding to theresult of detection of at least either one of a perpendicularity errorof the first array from a design value and a mean value of rotationangles of the shot areas in the first array.

In the above-described exposure method according to the presentinvention, a scanning type exposure apparatus such as a step-and-scanexposure apparatus is used as the second exposure apparatus. Let usassume that the width in one direction (e.g. a direction Y) of each shotarea in the first array is L, and that the first array has aperpendicularity error W. Further, the width in the direction Y of eachshot area in the second array formed by the second exposure apparatus isassumed to be (3/2)L. In this case, if exposure is carried out with thecenters of shot areas in the second array being merely aligned with thecenter line of the first array, an overlay error of a predeterminedmaximum width occurs between the first and second mask pattern images.

Therefore, in the above-described method according to the presentinvention, exposure is carried out with the centers of shot areas in thesecond array being displaced relative to the center line of the firstarray in a direction perpendicular to the direction Y by a width d(≈L·W/4). By doing so, the overlay error between the first mask patternimage and the second mask pattern image on the substrate is reduced to aminimum on the average. In a case where the first array has a shotrotation also, the overlay error can be minimized by displacing theposition of each shot area in the second array.

Further, if the rotation that is used in the first exposure method isused in the second exposure method, the overlay error is furtherreduced.

In addition, the present invention provides another exposure method inwhich a first mask pattern is transferred onto a photosensitivesubstrate in the form of a predetermined array by using a first exposureapparatus having a first exposure field of a predetermined shape, and asecond mask pattern is transferred onto the photosensitive substrateover the first mask pattern array by using a second exposure apparatushaving a second exposure field different from the first exposure fieldin length in a predetermined direction. In the exposure method, when thefirst mask pattern is to be transferred onto the photosensitivesubstrate by using the first exposure apparatus, an array of a pluralityof shot areas to each of which the first mask pattern is to betransferred is set on the photosensitive substrate along a direction(X1) corresponding to the direction in which the first exposure field isdifferent in length from the second exposure field.

According to the above-described exposure method of the presentinvention, the array of a plurality of shot areas to which the firstmask pattern is to be transferred is such that an imaginary straightline passing through shot areas which are adjacent to each other in thedirection X1, which corresponds to the direction in which the firstexposure field is different in length from the second exposure field, isparallel to the direction X1. As a result, when shot areas of a secondlayer are arrayed over the shot areas of the first layer by using thesecond exposure apparatus, the overlay error is minimized even if theshot array of the first layer has a perpendicularity error.

In this exposure method, when the first mask pattern is to betransferred onto the photosensitive substrate by using the firstexposure apparatus, the photosensitive substrate and the first maskpattern have previously been rotated through 90° from their ordinarypositions. By doing it so, even if a perpendicularity error is presentin the shot array of the first layer, the shot areas of the first layercan be arrayed in a straight-line form along the direction (X1)corresponding to the direction in which the first exposure field isdifferent in length from the second exposure field.

One example of the second exposure apparatus is a scanning exposure typeexposure apparatus; in this case, it is desirable that theabove-described predetermined direction should be the scanningdirection. The reason for this is that the exposure field of a scanningexposure type exposure apparatus can be readily lengthened in thescanning direction.

In addition, the present invention provides another exposure method inwhich mask patterns are overlaid on one another on a photosensitivesubstrate, which is an object to be exposed, by using a first exposureapparatus having a first exposure field of a predetermined size on thephotosensitive substrate, and a second exposure apparatus having asecond exposure field which is M₁/N₁ times (M₁ and N₁ are integers;M₁>N₁) as large as the first exposure field in a first direction andwhich is M₂/N₂ times (M₁ and N₂ are integers; M₂≧N₂) as large as thefirst exposure field in a second direction which is perpendicular to thefirst direction. The exposure method has the first step of sequentiallytransferring an image of a first mask pattern, which has an alignmentmark and a first overlay accuracy measuring mark, onto thephotosensitive substrate in the form of a two-dimensional arrayextending in the first and second directions in units of the firstexposure field by using the first exposure apparatus.

The exposure method according to the present invention further has: thesecond step of transferring an image of a second mask pattern, which hasa second overlay accuracy measuring mark, over a plurality of images ofthe first mask pattern, which have been transferred onto thephotosensitive substrate in the first step, in a two-dimensional arrayextending in the first and second directions on the photosensitivesubstrate in units of the second exposure field with reference to theposition of the image of the alignment mark by using the second exposureapparatus; and the third step of dividing an exposure area on thephotosensitive substrate into a plurality of reference measurement areasin units of an area which is N₁ times as large as the width of thesecond exposure field in the first direction on the photosensitivesubstrate and which is N₂ times as large as the width of the secondexposure field in the second direction on the photosensitive substrate,and measuring an amount of positional displacement between the images ofthe first and second overlay accuracy measuring marks lying at themutually identical positions in a predetermined number of referencemeasurement areas selected from among the plurality of referencemeasurement areas, thereby obtaining a correction value which is usedwhen the position of the alignment mark image transferred by the firstexposure apparatus is detected by the second exposure apparatus on thebasis of the amount of positional displacement measured as described Haabove. Thereafter, the exposure position is corrected by using thecorrection value obtained in the third step when overlay exposure iscarried out by using the second exposure apparatus with respect to thesurface of the photosensitive substrate exposed by the first exposureapparatus.

In this case, it is desirable for the second exposure apparatus tocalculate the exposure position on the basis of the alignment mark imageand by use of a predetermined coordinate transformation parameter (EGAparameter) and to obtain a correction value for the coordinatetransformation parameter in the third step.

According to the above-described exposure method of the presentinvention, the first exposure apparatus is used, for example, forexposure of a critical layer, and the second exposure apparatus is used,for example, for exposure of a middle layer because the second exposurefield is larger than the first exposure field. Further, because thesecond exposure field is M₁/N₁ times and M₂/N₂ times as large as thefirst exposure field in the first and second directions, respectively,if the widths in the first and second directions of the first maskpattern image formed by the first exposure apparatus are denoted by dand c, respectively, the widths in the first and second directions ofthe second mask pattern image formed by the second exposure apparatusare dM₁/N₁ and cM₂/N₂, respectively.

Assuming that the integers M₁ and N₁ have no common divider other than1, and the integers M_(2 and N) ₂ also have no common divider other than1, an area on the photosensitive substrate which has a size regarded asbeing the least common multiple of the sizes of the first and secondmask pattern images is an area which has a width dM₁ in the firstdirection and a width cM₂ in the second direction, that is, a referencemeasurement area which is N₁ times and N₁ times as large as the width ofthe second exposure field in the first and second directions,respectively. Such a reference measurement area contains an integernumber of first and second mask pattern images in each of the first andsecond directions.

If M₁=2, N₁=1, M₂=2, and N₁=1, for example, the second mask patternimage itself is the reference measurement area. In such a case, in thepresent invention, if an amount of positional displacement between twocorresponding alignment mark images is measured at a measuring point inthe top right portion of the first reference measurement area, an amountof positional displacement between two corresponding alignment markimages is similarly measured at a measuring point in the top rightportion of each of the second to fourth reference measurement areas.Thus, a magnification error or rotation error of the second exposurefield is approximately equally introduced into all the measured amountsof positional displacement. Accordingly, there is no likelihood that amagnification error or rotation error of the second exposure field willbe mistaken for an error component other than an offset component in theamount of positional displacement between the first and second maskpattern images. Thus, the overlay accuracy improves.

In this case, if an alignment method in which coordinate transformationparameters are employed, e.g. the EGA method, is used for the exposureprocess carried out by the second exposure apparatus, there is nolikelihood that a magnification error or rotation error of the secondexposure field will be mistaken for a coordinate transformationparameter other than an offset.

In addition, the present invention provides another exposure method inwhich mask patterns are overlaid on one another on a photosensitivesubstrate by using a first and second exposure apparatuses havingrespective exposure fields of different sizes. In the exposure method,images of a first and second mask patterns containing overlay accuracymeasuring marks are sequentially transferred onto a photosensitivesubstrate for evaluation, being overlaid on one another, by using thefirst and second exposure apparatuses, and an amount of positionaldisplacement between the overlaid images of the overlay accuracymeasuring marks is measured at a predetermined measuring point in areference measurement area on the evaluation photosensitive substrate inwhich a shot area formed in units of the exposure field of the firstexposure apparatus and a shot area formed in units of the exposure fieldof the second exposure apparatus are overlaid on one another such thatneither of the overlaid shot areas extends over beyond a part of thereference measurement area (or neither of the overlaid shot areasextends over a plurality of shot areas). On the basis of the result ofthe measurement, alignment or correction of image-formationcharacteristics is effected when exposure is to be carried out by thesecond exposure apparatus with respect to the surface of thephotosensitive substrate exposed by the first exposure apparatus.

In the above-described exposure method according to the presentinvention, the reference measurement area does not extend over aplurality of shot areas in either of two layers. Therefore, the amountof positional displacement measured at a measuring point in thereference measurement area contains no effect of stepping error ofeither of the exposure apparatuses. Accordingly, a high overlay accuracyis obtained by carrying out exposure after a parameter for alignment ora parameter indicating image-formation characteristics has beencorrected on the basis of the measured amount of positionaldisplacement.

In addition, the present invention provides another exposure method inwhich mask patterns are overlaid on one another on a photosensitivesubstrate, which is an object to be exposed, by using a first exposureapparatus having a first exposure field of a predetermined size on thephotosensitive substrate, and a second exposure apparatus having asecond exposure field which is M₁/N₁ times (M₁ and N₁ are integers;M₁≈N₁) as large as the first exposure field in a first direction andwhich is M₂/N₂ times (M₂ and N₂ are integers) as large as the firstexposure field in a second direction which is perpendicular to the firstdirection. The exposure method has: the first step of sequentiallytransferring an image of a first mask pattern, which has an alignmentmark and a first overlay accuracy measuring mark, onto a plurality offirst shot areas arrayed on the photosensitive substrate in units of thefirst exposure field by using the first exposure apparatus; and thesecond step of sequentially transferring an image of a second maskpattern, which has a second overlay accuracy measuring mark, onto aplurality of second shot areas arrayed on the photo-sensitive substrate,exposed in the first step, in units of the second exposure field withreference to the image of the alignment mark by using the secondexposure apparatus.

Further, the exposure method according to the present invention has thethird step of defining a plurality of reference measurement areas on thephotosensitive substrate in each of which any one of the first shotareas and any one of the second shot areas are overlaid on one anothersuch that neither of the overlaid shot areas extends over beyond a partof the reference measurement area, and measuring an amount of positionaldisplacement between the images of the first and second overlay accuracymeasuring marks lying at the mutually identical positions in apredetermined number of reference measurement areas selected from amongthe plurality of reference measurement areas, thereby obtaining acorrection value which is used when the position of the alignment markimage transferred by the first exposure apparatus is detected by thesecond exposure apparatus on the basis of the amount of positionaldisplacement measured as described above.

In this case, the first exposure apparatus is used, for example, for acritical layer, and the second exposure apparatus is used, for example,for a middle layer. In the exposure method, if a plurality of measuringpoints for measuring an amount of positional displacement between a pairof overlay accuracy measuring marks are set in each referencemeasurement area in order to obtain a linear expansion and contractionerror or the like in a shot area, the distribution of the measuringpoints does not extend over a plurality of first shot areas nor aplurality of second shot areas. Accordingly, the linear expansion andcontraction error or the like can be accurately obtained without beingaffected by stepping errors of the critical and middle layers, and theoverlay accuracy between the critical and middle layers is improved bycorrecting the linear expansion and contraction error at the middlelayer.

In this case, each alignment mark and each first overlay accuracymeasuring mark may be the same mark.

One example of the correction value obtained in the third step is acorrection value for a parameter indicating a predeterminedimage-formation characteristic calculated on the basis of the positionsof the alignment mark images. One example of such a parameter is atleast one parameter selected from a parameter group consisting of shotmagnifications rx and ry, shot rotation θ, and shot perpendicularity w.In this case, it is desirable to correct the image-formationcharacteristic by using the correction value obtained in the third stepwhen overlay exposure is to be carried out thereafter by using thesecond exposure apparatus with respect to the surface of thephotosensitive substrate exposed by the first exposure apparatus.

Further, one example of the first exposure apparatus is a one-shotexposure type projection exposure apparatus, and one example of thesecond exposure apparatus is a scanning exposure type projectionexposure apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing an exposure systemused in a first example of a first embodiment of the exposure methodaccording to the present invention.

FIG. 2(a) illustrates the detection principle of a laser step alignmenttype alignment system.

FIG. 2(b) is an enlarged view showing one example of a wafer mark whichis used in another type of alignment system.

FIG. 3 is a plan view showing a shot array of a critical layer on awafer in the first example.

FIG. 4 is an enlarged plan view showing a part of the shot array shownin FIG. 3, together with a shot area of a middle layer exposed over theshot array.

FIG. 5 is a plan view showing a shot array of a middle layer exposedover the shot array shown in FIG. 3 in the first example.

FIG. 6(a) is a plan view of a second example of the first embodiment ofthe present invention, showing an array of shot areas exposed on a firstlayer, together with one example of a shot array in a case where shotareas exposed over the first-layer shot areas have a large overlayerror.

FIG. 6(b) is a plan view showing a shot area exposed by a scanning typesecond exposure apparatus.

FIG. 7 is a plan view showing an array of shot areas in which theoverlay error reduces in the second example of the first embodiment.

FIGS. 8(a), 8(b) and 8(c) illustrate an alignment method for overlayexposure in which short shot areas are overlaid on an array of long shotareas in the second example of the first embodiment.

FIG. 9(a) is a plan view of a third example of the first embodiment ofthe present invention, showing a shot array on a wafer.

FIG. 9(b) is a plan view showing a shot area exposed by a secondexposure apparatus.

FIG. 10(a) is a plan view showing a part of the shot array shown in FIG.9(a), together with one example of an array in a case where shot areasexposed over the shot array have a large overlay error.

FIG. 10(b) is a plan view showing an array of shot areas in which theoverlay error reduces.

FIG. 10(c) illustrates an alignment method for overlay exposure in whichshort shot areas are overlaid on an array of long shot areas.

FIG. 11 is a perspective view schematically showing an exposure systemused in a first example of a second embodiment of the exposure methodaccording to the present invention.

FIG. 12(a) is a plan view showing the orientation of a reticle whenexposure is carried out for a first layer on a wafer in the firstexample of the second embodiment.

FIG. 12(b) is a plan view showing the orientation of a wafer whenexposure is carried out for the first layer on the wafer.

FIG. 13 is a plan view for explaining an alignment method executedbefore exposure is carried out for the second layer on the wafer in thefirst example of the second embodiment.

FIG. 14(a) in a plan view showing a shot array when exposure is carriedout for a second layer on a wafer in the first example of the secondembodiment.

FIG. 14(b) is a plan view showing the orientation of a reticle whenexposure is carried out for the second layer.

FIG. 14(c) is a plan view showing first-layer shot areas on the wafer.

FIG. 15 is a plan view illustrating a second example of the secondembodiment, showing a wafer which is placed on a wafer stage of ascanning type exposure apparatus after it has been subjected to exposurefor a first layer.

FIG. 16 is a plan view showing a shot array of a second layer on thewafer in the second example of the second embodiment.

FIG. 17(a) is a plan view of a third example of the second embodiment,showing shot arrays of first and second layers on a wafer.

FIG. 17(b) is a plan view showing the orientation of a reticle whenexposure is carried out for the second layer on the wafer.

FIG. 17(c) is a plan view showing the orientation of a reticle whenexposure is carried out for the first layer on the wafer.

FIG. 18 is a perspective view schematically showing an exposure systemused in a third embodiment of the exposure method according to thepresent invention.

FIG. 19(a) is a plan view showing a shot array of a critical layer on awafer in the third embodiment.

FIG. 19(b) is an enlarged plan view showing an arrangement of verniermarks in a shot area of the critical layer.

FIG. 20(a) is a plan view showing a shot array and measuring pointarrangement on a middle layer exposed over the critical layer shown inFIG. 19(a).

FIG. 20(b) is an enlarged plan view showing a part of the vernier markarrangement in a shot area of the middle layer.

FIGS. 21(a) and 21(b) show another example of the arrangement ofmeasuring points on the wafer.

FIG. 22(a) is an enlarged view showing another example of thearrangement of measuring points in a shot area of the middle layer.

FIG. 22(b) is an enlarged view showing still another example of thearrangement of measuring points in a shot area of the middle layer.

FIGS. 23(a) and 23(b) are plan views each showing one example of adesirable arrangement of measuring points on a wafer.

FIGS. 24(a) and 24(b) show one example of an arrangement of measuringpoints in a case where measuring points are selected from those whichare at different positions in a plurality of reference measurement areason a wafer.

FIGS. 25(a), 25(b) and 25(c) show an example of reference measurementareas used in a case where a plurality of chip patterns fit in each shotarea of a critical layer.

FIGS. 26(a), 26(b) and 26(c) show an example of reference measurementareas used in a case where shot areas of a middle layer are exposed by ascanning exposure method.

FIGS. 27(a) and 27(b) show that the expansion and contraction quantityof shot areas of a middle layer differs according to measuring points inthe example shown in FIGS. 26(a) to 26(c).

FIGS. 28(a), 28(b) and 28(c) show an example of reference measurementareas used in a case where shot areas of a middle layer are exposed by ascanning exposure method, and shot areas of a critical layer are widerthan the middle layer shot areas.

FIG. 29 is a perspective view schematically showing an exposure systemused in a fourth embodiment of the exposure method according to thepresent invention.

FIG. 30(a) is a plan view showing a shot array of a critical layer on awafer in the fourth embodiment of the present invention.

FIG. 30(b) is an enlarged plan view showing an arrangement of verniermarks in a shot area of the critical layer shown in FIG. 30(a).

FIG. 31(a) is a plan view showing a shot array of a middle layer exposedover the critical layer shown in FIG. 30(a).

FIG. 31(b) is an enlarged plan view showing vernier mark arrangements ontwo layers in a shot area of the middle layer shown in FIG. 31(a).

FIG. 32 is an enlarged plan view showing one example of referencemeasurement areas and measuring points set on the wafer shown in FIG.31(a).

FIGS. 33(a), 33(b) and 33(c) show the way in which reference measurementareas are determined in a case where a middle layer shot area is twiceas large as a critical layer shot area in each of directions X and Y.

FIGS. 34(a), 34(b) and 34(c) show the way in which reference measurementareas are determined in a case where first-layer shot areas are exposedby a one-shot exposure method, while second-layer shot areas are exposedby a scanning exposure method, and the first-layer shot areas are widerthan the second-layer shot areas.

FIG. 35 is a view for explanation of a background art related to thepresent invention, showing an overlay error due to a perpendicularityerror of the shot array on the preceding layer.

FIG. 36 is a view for explanation of a background art related to thepresent invention, showing an overlay error due to a shot rotation ofthe shot array on the preceding layer.

FIGS. 37(a), 37(b) and 37(c) are views for explanation of a backgroundart related to the present invention, showing one example of amix-and-match exposure process.

FIGS. 38(a), 38(b) and 38(c) are views for explanation of a backgroundart related to the present invention, showing a case where an overlayerror arises in mix-and-match exposure process.

FIG. 39(a) is a plan view of a background art related to the presentinvention, showing a shot array of a critical layer on a wafer.

FIG. 39(b) is an enlarged plan view showing an arrangement of verniermarks in a shot area of the critical layer shown in FIG. 39(a).

FIG. 40(a) is a plan view of a shot array and measuring pointarrangement on a middle layer exposed over the critical layer shown inFIG. 39(a).

FIG. 40(b) is an enlarged plan view showing an arrangement of verniermarks in a shot area of the middle layer shown in FIG. 40(a).

FIGS. 41(a) and 41(b) illustrate a background art related to the presentinvention, in which FIG. 41(a) shows a shot area of a middle layer whichhas a magnification error, and FIG. 41(b) shows a middle layer shot areawhich has a rotation error.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first example of a first embodiment of the exposure method accordingto the present invention will be described below with reference to FIGS.1 to 5. In this embodiment, two exposure apparatuses are used: a firstexposure apparatus of the stepper type (one-shot exposure type) with ademagnification ratio of 5:1, and a second exposure apparatus of thestepper type with a demagnification ratio of 2.5:1. In this case, oneshot area exposed by the second exposure apparatus corresponds to fourshot areas exposed by the first exposure apparatus.

FIG. 1 shows an exposure system used in an exposure method according tothe first embodiment of the present invention. In the exposure systemshown in FIG. 1 are installed a first exposure apparatus 1A of thestepper type which has a small exposure field, and a second exposureapparatus 1B of the stepper type which has a large exposure field. Inthis embodiment, the exposure apparatus 1A is a high-resolution exposureapparatus, while the exposure apparatus 1B is a low-resolution exposureapparatus. The high-resolution exposure apparatus 1A is used to carryout exposure for a critical layer on a wafer, and the low-resolutionexposure apparatus 1B is used to carry out exposure for a middle layeron the wafer. However, the exposure apparatus 1A may be a low-resolutionexposure apparatus or the exposure apparatus 1B may be a high-resolutionexposure apparatus according to the kind of semiconductor device to beproduced.

First, in the exposure apparatus 1A, a pattern area 2A on a reticle RAis illuminated by exposure light from an illumination optical system(not shown), and an image of a pattern formed in the pattern area 2A isformed on an exposure field 4A on a wafer 20 as a projected imagereduced to ⅕ by a projection optical system 3A. A Z1-axis is taken in adirection parallel to an optical axis of the projection optical system3A, and two axes of an orthogonal coordinate system set in a planeperpendicular to the Z1-axis are defined as an X1-axis and a Y1-axis,respectively. The reticle RA has an alignment mark 17X for the X1-axisformed at an end of the pattern area 2A in the direction Y1 (e.g. withina masking frame) and also has an alignment mark 17Y for the Y1-axisformed at an end of the pattern area 2A in the direction X1.

The wafer 20 is held on a wafer stage 5A. The wafer stage 5A comprises aZ-stage for moving the wafer 20 in the direction Z1 to set an exposuresurface of the wafer 20 which is to be exposed at the best focusposition, and an XY-stage for positioning the wafer 20 in both thedirections of the X1- and Y1-axes. A pair of moving mirrors 6A and BAwhich are perpendicular to each other are fixed on the wafer stage 5A.The coordinate in the direction X1 of the wafer stage 5A is measured bya combination of the moving mirror 6A and a laser interferometer 7Awhich is installed outside the wafer stage 5A. The coordinate in thedirection Y1 of the wafer stage 5A is measured by a combination of themoving mirror 8A and a laser interferometer 9A which is installedoutside the wafer stage 5A. The coordinates measured by the laserinterferometers 7A and 9A are supplied to a controller 10A whichcontrols operations of the whole apparatus. The controller 10A drivesthe wafer stage 5A to step in both the directions X1 and Y1 throughdrive units (not shown), thereby positioning the wafer 20. In this case,the stepping drive of the wafer 20 is effected according to an array ofshot areas (i.e. unit areas to each of which a pattern image of thepattern area 2A is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a critical layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10A.

The exposure apparatus 1A is provided with alignment systems 11A and 14Aboth of which are TTL (Through-The-Lens) and laser step alignment type(hereinafter referred to as “LSA type”) systems. An LSA type alignmentsystem is disclosed in detail, for example, in JP(A) No. 60-130742.Therefore, only an outline of the alignment systems 11A and 14A will begiven below. A laser beam emitted from the alignment system 11A for theX1-axis is reflected by a mirror 12A, which is disposed between theprojection optical system 3A and the reticle RA, and the reflected laserbeam enters the projection optical system 3A. The laser beam emanatingfrom the projection optical system 3A is converged onto an area near theexposure field 4A in the form of a slit-shaped light spot 13A elongatedin the direction Y1.

FIG. 2(a) shows a wafer mark MX for the X1-axis which serves as analignment mark on the wafer 20, which is an object to be exposed. InFIG. 2(a), the wafer mark MX is a dot train pattern comprising recessesand projections, which are arranged at a predetermined pitch in adirection approximately parallel to the slit-shaped light spot 13A. Whenthe wafer mark MX is scanned in the direction X1 relative to theslit-shaped light spot 13A by driving the wafer stage 5A, shown in FIG.1, diffracted light is emitted in a predetermined direction as the wafermark MX coincides with the slit-shaped light spot 13A.

Referring to FIG. 1, the diffracted light returns to the alignmentsystem 11A via the projection optical system 3A and the mirror 12A. Inthe alignment system 11A, the diffracted light is photoelectricallyconverted by a light-receiving element to obtain an alignment signal.The alignment signal is supplied to the controller 10A. In thecontroller 10A, the X1 coordinate of the wafer stage 5A measured whenthe alignment signal reaches a maximum, for example, is sampled, therebydetecting the position of the wafer mark MX in the direction of theX1-axis.

Similarly, a laser beam emitted from the LSA type alignment system 14Afor the Y1-axis enters the projection optical system 3A via a mirror 15Aand is converged onto the wafer 20 in the form of a slit-shaped lightspot 16A elongated in the direction of the X1-axis. Diffracted lightgenerated from the slit-shaped light spot 16A returns to the alignmentsystem 14A via the projection optical system 3A and the mirror 15A. Thealignment system 14A supplies an alignment signal to the controller 10A.Thus, the position in the Y1-axis direction of a wafer mark for theY1-axis on the wafer 20 is detected on the basis of the alignmentsignal.

It should be noted that, as each of the alignment systems 11A and 14A,it is also possible to use a TTL (Through-The-Lens) type alignmentsystem or an off-axis type alignment system which detects the positionof a wafer mark without passing a wafer mark detecting light beamthrough the projection optical system 3A. As a wafer mark detectingmethod, it is also possible to use an image processing type detectionmethod, or a so-called two-beam interference type detection method inwhich two light beams are applied to a diffraction grating-shaped wafermark, and the position of the wafer mark is detected from a signalobtained from interference between a pair of diffracted light beamsgenerated in parallel from the illuminated wafer mark. When such animage processing type or two-beam interference type alignment system isused, a line-and-space pattern 22X as shown in FIG. 2(b) is used. Theline-and-space pattern 22X comprises recesses and projections, which arearranged at a predetermined pitch in the measuring direction, forexample.

Next, the second exposure apparatus 1B will be explained. The exposureapparatus 1B has an arrangement approximately similar to that of theabove-described first exposure apparatus 1A. An image of a patternformed in a pattern area 2B of a reticle RB is projected through aprojection optical system 3B onto an exposure field 4B on a wafer 20held on a wafer stage 5B as an image reduced to {fraction (1/2.5)}. AZ2-axis is taken in a direction parallel to an optical axis of theprojection optical system 3B, and two axis of an orthogonal coordinatesystem set in a plane perpendicular to the Z2-axis are defined as anX2-axis and a Y2-axis, respectively. The reticle RB has the pattern areadivided into two columns in the direction X2 and two rows in thedirection Y2 to form partial pattern areas 18A to 18D. The partialpattern areas 18A to 18D each has the same circuit pattern formedtherein. Further, the partial pattern areas 18A to 18D are each providedwith the same alignment mark 19X for the X2-axis and the same alignmentmark 19Y for the Y2-axis. The X2 coordinate of the wafer stage 5B ismeasured by a combination of a moving mirror 6B and a laserinterferometer 7B. The Y2 coordinate of the wafer stage 5B is measuredby a combination of a moving mirror 8B and a laser interferometer 9B.The measured coordinates are supplied to a controller 10B. Thecontroller 10B controls the stepping drive of the wafer stage 5B.

The stepping drive of the wafer stage 5B is effected according to anarray of shot areas (i.e. areas to each of which a pattern image of thepattern area 2B is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a middle layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10B. In this case, the map generating unit inthe controller 10A and the map generating unit in the controller 10Bhave the function of supplying shot map information prepared thereby toeach other. When exposure for a middle layer is to be carried out over acritical layer, for example, shot map information for the critical layerprepared by the map generating unit in the controller 10A of theexposure apparatus 1A is transmitted from a communication unit in thecontroller 10A to a communication unit in the controller 10B. The mapgenerating unit in the controller 10B generates a shot map for themiddle layer on the basis of the supplied shot map information.Conversely, when exposure for a critical layer is to be carried out overa middle layer, shot map information for the middle layer prepared bythe map generating unit in the controller 10B is supplied to the mapgenerating unit in the controller 10A.

In the exposure apparatus 1B also, an alignment system 11B for theX2-axis is a TTL and LSA type alignment system. A laser beam from thealignment system 11B enters the projection optical system 3B via amirror 12B. The laser beam is converged through the projection opticalsystem 3B onto the wafer 20 in the form of a slit-shaped light spot 13Belongated in the direction Y2. A laser beam from an alignment system 14Bfor the Y2-axis enters the projection optical system 3B via a mirror15B, and the laser beam is converged through the projection opticalsystem 3B onto the wafer 20 in the form of a slit-shaped light spot 16Belongated in the direction X2. Diffracted light beams from theslit-shaped light spots 13B and 16B are received by the correspondingalignment systems 11B and 14B, thereby detecting the positions of thewafer marks for the Y2- and X2-axes on the wafer 20.

Next, an exposure method in this embodiment will be explained withreference to FIGS. 3 to 5. In this embodiment, the exposure process willbe explained by way of an example in which a pattern image of a reticlefor a middle layer is transferred by using the second exposure apparatus1B over a critical layer transferred on the wafer 20 by using the firstexposure apparatus 1A.

FIG. 3 shows a shot array of a critical layer on the wafer 20. in FIG.3, the surface of the wafer 20 is divided into square shot areas SA₁₁,SA₁₂, . . . , SA₉₄ at a predetermined pitch in each of first and seconddirections. Each side of each square shot area SA has a length L. Theshot areas SA₁₁ to SA₉₄ have approximately the same size as that of theexposure field 4A of the exposure apparatus 1A, shown in FIG. 1. Animage of the circuit pattern in the pattern area 2A of the reticle RA isprojected onto each of the shot areas SA₁₁ to SA₉₄ by using the exposureapparatus 1A, shown in FIG. 1. By development and other processescarried out thereafter, the circuit pattern images are made to appear asreal circuit patterns. Further, each of the shot areas SA_(ij) (i−1 to9; j=1 to 4) is provided with images of the alignment marks 17X and 17Yformed on the reticle RA, shown in FIG. 1, as a wafer mark MX_(ij) forthe X-axis and a wafer mark MY_(ij) for the Y-axis.

Next, a photoresist is coated over the wafer 20. The wafer 20 coatedwith the photoresist is loaded onto the wafer stage 5B in the exposureapparatus 1B, shown in FIG. 1, and a circuit pattern image of thereticle RB is projected onto each of shot areas of a middle layer overthe critical layer on the wafer 20. In this case, each group of fourcritical layer shot areas arrayed in two rows and two columns as shownin FIG. 3 corresponds to one middle layer shot area. For example, agroup of four shot areas SA₁₁ to SA₁₄ in the top left corner correspondsto one middle layer shot area SB₁. To generate such a shot map for themiddle layer, the exposure apparatus 1B first effects EGA alignment. TheEGA alignment method is disclosed, for example, in JP(A) No. 4-277612 inaddition to JP(A) No. 61-44429.

Here, the X2- and Y2-axes of the coordinate system that define thetravel position of the wafer stage 5B of the second exposure apparatus1B, shown in FIG. 1, are taken as X- and Y-axes, respectively, in FIG.3, and a coordinate system that is defined by the X- and Y-axes isreferred to as “stage coordinate system (X,Y)”. From among the criticallayer shot areas SA₁₁ to SA₉₄ on the wafer 20 shown in FIG. 3, apredetermined number N (N is an integer of 3 or more) shot areas (i.e.shaded shot areas in the figure) are selected as sample shots S₁ to S₉(in this case, N=9), and coordinate values in the stage coordinatesystem (X,Y) of the wafer marks attached to the sample shots S₁ to S₉are measured by using the alignment systems 1B and 14B, shown in FIG. 1.For the sake of simplicity, it is assumed in the following descriptionthat the X coordinate of the X-axis wafer mark MX_(ij) attached to ashot area SA_(ij) and the Y coordinate of the Y-axis wafer mark MY_(ij)attached to the shot area SA_(ij) represent the X and Y coordinates ofthe center of the shot area SA_(ij).

Further, coordinate axes which constitute the coordinate system on thewafer 20 (i.e. sample coordinate system) are assumed to be an x-axis anda y-axis, respectively. It is further assumed that design coordinatevalues of the centers of the shot areas SA₁₁ to SA₉₄ on the criticallayer in the sample coordinate system (x,y) have already been suppliedto the controller 10B of the second exposure apparatus 1B as a part ofshot map data for tie critical layer. Under these circumstances, thetransformation of array coordinates of an arbitrary point on the wafer20 in the sample coordinate system (x,y) into array coordinates in thestage coordinate system (X,Y) is approximately expressed by thefollowing equation (3): $\begin{matrix}{\begin{bmatrix}X \\Y\end{bmatrix} = {{\begin{bmatrix}{Rx} & {- {{Rx}( {W + \theta} )}} \\{{Ry} \cdot \theta} & {Ry}\end{bmatrix}\begin{bmatrix}x \\y\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (3)\end{matrix}$

The transformation matrix in Eq. (3) has as elements six coordinatetransformation parameters, including scaling parameters Rx and Ry of thewafer, a rotation θ [rad] of the shot array, a perpendicularity error W[rad] of the shot array, and offsets Ox and Oy. The scaling parametersRx and Ry are linear expansion and contraction quantities of the waferin the directions X and Y, respectively. The rotation θ is an angle ofrotation of the x-axis of the sample coordinate system relative to theX-axis. The perpendicularity error W is an error of the intersectionangle between the x- and y-axes of the sample coordinate system from90°. The offsets Ox and Oy are shift quantities in the directions X andY, respectively.

Eq. (3) is usable in the present invention; in this embodiment, however,Eq. (3) is approximated with the following equation (4) using 1+Γx and1+Γy for the scaling parameters Rx and Ry and regarding the values ofthe new parameters Γx and Γy as small in order to facilitate thecalculation: $\begin{matrix}{\begin{bmatrix}X \\Y\end{bmatrix} = {{\begin{bmatrix}{1 + {\Gamma \quad x}} & {- ( {W + \theta} )} \\\theta & {1 + {\Gamma \quad y}}\end{bmatrix}\begin{bmatrix}x \\y\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (4)\end{matrix}$

To determine values of the six transformation parameters (Γx, Γy, θ, w,Ox and Oy) in Eq. (4), the controller 10B defines the array coordinatevalues of the centers (wafer marks) of sample shots S₁ measured by thei-th (i=1 to N) measuring operations as (XM_(i),YM_(i)). Next, thedesign array coordinates (x_(i),y_(i)) of the centers of the sampleshots S_(i) are substituted for the coordinates (x,y) on the right-handside of Eq. (4) to obtain computational array coordinate values(X_(i),Y_(i)). The sum of the squares of deviations of the measuredvalues (XM_(i),YM_(i)) from the array coordinate values (X_(i),Y_(i)) isdetermined to be a residual error component as expressed by thefollowing equation (5): $\begin{matrix}{{{Residual}\quad {error}\quad {component}} = {\sum\limits_{i = 1}^{N}\{ {( {X_{i} - {XM}_{i}} )^{2} + ( {Y_{i} - {YM}_{i}} )^{2}} \}}} & (5)\end{matrix}$

Then, the controller 10B determines values of the six transformationparameters so that the residual error component is minimized. Forexample, values of the six parameters are obtained by solvingsimultaneous equations established by setting the result of partialdifferentiation of the right-hand side of Eq. 5 with respect to each ofthe six parameters equal to zero.

In this embodiment, it is assumed that among the transformationparameters obtained as described above, the shot array rotation θ [rad]is regarded as zero, and the shot array perpendicularity error W [rad]assumes a other parameters, that is, scaling parameters Γx and Γy andoffsets Ox and Oy, may assume any values, respectively. In this case,the array of the critical layer shot areas SA₁₁ to SA₉₄ is as follows:As shown in FIG. 3, for example, an imaginary straight line 23connecting the centers of shot areas which are adjacent to each other inthe direction X is parallel to the X-axis. An imaginary straight line 24which passes through the center C₁₁ of the first shot area SA₁₁ andwhich connects the centers of shot areas which are successively adjacentto the shot area SA₁₁ in the direction Y has been rotated clockwiserelative to the Y-axis by the perpendicularity error W.

Next, in this embodiment, the reticle RB in the exposure apparatus 1B,shown in FIG. 1, is rotated through a predetermined angle δ [rad] tothereby rotate each shot area of the middle layer by an angle δ in orderto reduce an overlay error between the critical and middle layers due tothe perpendicularity error W.

FIG. 4 shows the positional relationship between four shot areas SA₁₁ toSA₁₄ of the critical layer and one middle layer shot area SB₁ over thefour critical layer shot areas. In FIG. 4, the shot area SB₁ has itsadjacent sides rotated through an angle δ clockwise from the respectivepositions which are parallel to the X- and Y-axes. Further, thecontroller 10 successively substitutes the design array coordinates ofthe four shot areas SA₁₁ to SA₁₄ and the above-determined transformationparameters into the right-hand side of Eq. (4), thereby obtaining centercoordinates of the four shot areas SA₁₁ to SA₁₄ in the stage coordinatesystem (X,Y), and further obtaining coordinates of the center 25 of thefour sets of center coordinates. Then, a circuit pattern image of thereticle RB is projected onto the shot area SB₁ with the center 25 madecoincident with the center of the exposure field 4B. As a result,exposure is carried out in a state where the center 25 of the array ofthe four shot areas SA₁₁ to SA₁₄ is coincident with the center of theshot area SB₁, and the shot area SB₁ has been rotated through the angleδ.

Similarly, as shown in FIG. 5, a circuit pattern image of the reticle RBis sequentially projected onto middle layer shot areas SB₂, SB₃, . . . ,SB₉ deployed over the critical layer on the wafer 20.

Let us conduct evaluation of the overlay error in this embodiment withreference to FIG. 4. In this embodiment, in an array of four square shotareas SA₁₁ to SA₁₄, each side of which has a length L, shot areas whichare adjacent to each other in the direction X lie such that an imaginarystraight line 23A connecting the centers of these shot areas is parallelto the X-axis, and shot areas which are adjacent to each other in thedirection Y lie such that an imaginary straight line 24 connecting thecenters of these shot areas intersects the Y-axis at an angle(perpendicularity error) W. Accordingly, assuming that an overlay errorbetween the array of critical layer four shot areas SA₁₁ to SA₁₄ and themiddle layer shot area SB₁ is Δ, and that the perpendicularity error Wand the rotation angle δ are small, the ranges of X and Y componentsΔ_(x) and Δ_(y) of the overlay error A concerning the shot area SA₁₁ areapproximately given by the following equation (6): $\begin{matrix}{{{( {1/2} ){L \cdot W}} - {L \cdot \delta}} \leq \Delta_{x} \leq {( {1/2} ){L \cdot W}\quad 0} \leq \Delta_{Y} \leq {L \cdot \delta}} & (6)\end{matrix}$

In this case, if the rotation angle δ is assumed to be zero as in therelated art shown in FIG. 35, the X component Δ_(x) of the overlay errorΔ is uniformly (½)L·W, and the Y component Δ_(y) is uniformly zero.Therefore, in order to reduce the overlay error to a lower level than inthe related art as a whole, the rotation angle δ should be set withinthe range given by the following equation (7):

O<δ<(½)W  (7)

The value of the rotation angle δ at which the overlay error reaches aminimum as a whole within the above range is (¼)W. That is, in thiscase, the ranges of the X and Y components are obtained from Eq. (6) asfollows:

(¼)L·W≦Δ_(x)≦(½)L·W

O≦Δ_(y)≦(¼)L·W

Thus, it becomes possible to regard the overlay error as minimum as awhole.

Although in the above-described embodiment the reticle-side part in theexposure apparatus 1B is rotated through the rotation angle δ, thewafer-side part may be rotated through −δ instead of rotating thereticle-side part. However, if the wafer-side part is rotated, the shotarea array on the critical layer also changes, and it is thereforenecessary to correct the critical layer shot array. That is, rotatingthe reticle-side part is advantageous because it is possible to omit acorrective calculation which would otherwise be required.

Although in this embodiment the perpendicularity error W of the shotarray is assumed to be not zero, it should be noted that an alignmentmethod similar to that in the above-described embodiment is alsoapplicable in a case where the perpendicularity error W is zero as inthe related art shown in FIG. 36 and the shot rotation θ of each of thecritical layer shot areas SA₁₁ to SA₉₄, shown in FIG. 3, is not zero.That is, the overlay error can be reduced as a whole by rotating thereticle-side part of the second exposure apparatus 1B, for example, suchthat each middle layer shot area is rotated through the angle (θ+δ′)relative to the corresponding array of four critical layer shot areas,using the angle δ′ in the range of O<δ′<(½)θ. In particular, if theangle δ′ is set at (¼)θ, the overlay error is minimized as a whole. Themethod of detecting the shot rotation θ will be explained in a secondexample of the first embodiment.

Next, the second example of the first embodiment of the presentinvention will be described with reference to FIGS. 6(a) to 6(c). In thesecond example, the stepper type exposure apparatus 1A, shown in FIG. 1,is used as a first exposure apparatus, and a step-and-scan type scanningexposure apparatus with a demagnification ratio of 4:1 is used as asecond exposure apparatus. In the first exposure apparatus, an image oftwo identical circuit patterns (chip patterns) is transferred per shotarea; in the second exposure apparatus, an image of three identicalcircuit patterns is transferred per shot area.

FIG. 6(a) shows a part of a shot array on a wafer loaded on a waferstage (not shown) of the scanning type second exposure apparatus in thisexample. In FIG. 6(a), square shot areas SA₁, SA₂ and SA₃, each side ofwhich has a length L, are sequentially arrayed, lying adjacent to eachother in the direction Y. The shot areas SA₁, SA₂ and SA₃ each has twoidentical circuit patterns 26A and 26B formed side-by-side in thedirection Y by the first exposure apparatus, a developer, etc. In FIG.6(a), X- and Y-axes represent a stage coordinate system of the secondexposure apparatus. An imaginary straight line 27 passing through thecenters of the shot areas SA₁, SA₂ and SA₃ on the first layer is tiltedby an angle W clockwise relative to the Y-axis. The angle W is aperpendicularity error of the shot array.

FIG. 6(b) shows a shot area SC which has a width L in the direction Xand a width (3/2)L in the direction Y on a wafer which is to be exposedby the second exposure apparatus in this example. In FIG. 6(b),directions +Y and −Y are scanning directions. That is, the shot area SCis scanned in the direction +Y, for example, relative to a slit-shapedillumination field 28, and a reticle placed through a projection opticalsystem is scanned in the direction −Y in synchronism with the scanningof the shot area SC. As a result, three identical circuit pattern images29A to 29C are formed on the shot area SC, lying side-by-side in thedirection Y. In this example, it is assumed that circuit pattern imagesfor two shot areas SC₁ and SC₂ each having the same size as that of theshot area SC shown in FIG. 6(b) are overlaid on the shot areas SA₁, SA₂and SA₃ shown in FIG. 6(a) by using the second exposure apparatus.

In this case, it is conceivable to effect alignment such that, as shownby the chain double-dashed lines in FIG. 6(a), reference points 27 a and27 b, which are at the centers of two arrays of three circuit patternsarranged in the direction Y, coincide with the centers of second-layershot areas SC₁ and SC₂ respectively, on an imaginary straight line 27passing through the centers of the first-layer shot areas SA₁, SA₂ andSA₃. However, this alignment method causes overlay errors a and b in thedirection X between the second-layer shot area SC₁ and the first-layershot areas SA₁ and SA₂. Similarly, overlay errors b and a in thedirection X arise between the second-layer shot area SC₂ and thefirst-layer shot areas SA₂ and SA₃. The overlay errors a and b areexpressed by the following equation (8): $\begin{matrix}{a = {{( {1/4} ){L \cdot W}\quad b} = {( {3/4} ){L \cdot W}}}} & (8)\end{matrix}$

Accordingly, it will be understood that the alignment method shown inFIG. 6(a) causes a large overlay error b to arise particularly in thesecond shot area SA₂. In order to reduce the overlay error, in thisexample, the center positions of the second-layer shot areas SC₁ and SC₂are shifted-by a predetermined distance in the direction X from thereference points 27 a and 27 b, respectively.

FIG. 7 shows the alignment method according to this example. In FIG. 7,the center positions of the second-layer shot areas SC₁ and SC₂ areshifted by a distance d in the respective directions −X and +X relativeto the reference points 27 a and 27 b on the imaginary straight line 27passing through the centers of the first-layer shot areas SA₁ to SA₃. Asa result, there is a uniform overlay error c in the direction X betweenthe first-layer shot areas SA₁ to SA₃ and the second-layer shot areasSC₁ and SC₂. The distance d and the overlay error c are given by thefollowing equation (9): $\begin{matrix}{d = {{( {1/4} ){L \cdot W}\quad c} = {( {1/2} ){L \cdot W}}}} & (9)\end{matrix}$

As a result, the overlay error c given by Eq. (9) is (½)L·W in contrastto the overlay error b given by Eq. (8). Accordingly, it will beunderstood that the alignment method according to this example enablesthe maximum value of the overlay error to reduce to (½)L·W, and thus theoverlay error reduces as a whole. It should be noted that during thealignment shown in FIG. 7, the second-layer shot areas SC₁ and SC₂ maybe rotated through a predetermined angle relative to the first-layershot areas SA₁ to SA₃ by additionally applying the method according tothe first example. By doing so, the overlay error may be further reducedas a whole.

In the second example of the first embodiment, exposure may be carriedout by using the first exposure apparatus over shot areas exposed byusing the scanning type second exposure apparatus in reverse relation tothe above-described exposure operation. One example of such an exposureoperation will be explained below with reference to FIGS. 8(a), 8(b) and8(c).

FIG. 8(a) shows the first-layer shot areas SC₁ and SC₂ on the waferwhich have been formed with circuit patterns by using the scanningsecond exposure apparatus. In FIG. 8(a), the shot areas SC₁ and SC₂ eachhas three identical circuit patterns arranged in the direction Y, andthere is a predetermined perpendicularity error in the shot array. Then,a reticle pattern image including two identical circuit pattern imagesarranged in the direction Y is formed over the shot areas SC₁ and SC₂for each of the shot areas SA₁, SA₂ and SA₃ by using the first exposureapparatus. In this case, for the first and third shot areas SA₁ and SA₃it is only necessary to align them with the first-layer shot areas SC₁and SC₂, respectively, in the direction X. For the second shot area SA₂,it is only necessary to align its position in the direction X with anintermediate position between the first-layer shot areas SC₁ and SC₂.Consequently, the overlay error between the first and second layers is bonly at the shot area SA₂.

However, in a case where a reticle pattern image is transferred by usingthe first exposure apparatus, if a part of the pattern image to betransferred can be selectively masked by using a reticle blind (variablefield stop), for example, the overlay error can be reduced toapproximately zero. In such a case, as shown in FIG. 8(b), when thesecond shot area SA₂ of the second layer is to be exposed, first, theposition in the direction X of the shot area SA₂ is aligned with thefirst-layer shot area SC₁. Thereafter, exposure is carried out with thelower half of the shot area SA₂ masked by controlling the reticle blind.Consequently, exposure is effected only for the upper half of the shotarea SA₂, which corresponds to the circuit pattern 26A.

Next, the position in the direction X of the shot area SA₂ is alignedwith the first-layer shot area SC₂, and thereafter, exposure is carriedout with the upper half of the shot area SA₂ masked by controlling thereticle blind. Consequently, exposure is effected only for the lowerhalf of the shot area SA₂, which corresponds to the circuit pattern 26B.For the other shot areas SA₁ and SA₃, exposure similar to that in thecase of FIG. 8(a) is carried out. As a result, the overlay error becomeszero at all the shot areas.

Next, a third example of the first embodiment of the present inventionwill be described with reference to FIGS. 9(a) to 10(c). In thisexample, the stepper type exposure apparatus 1A, shown in FIG. 1, isused as a first exposure apparatus, and a step-and-scan type scanningexposure apparatus with a demagnification ratio of 4:1 is used as asecond exposure apparatus. In this case, shot areas as exposure unitswhich are to be exposed by the first exposure apparatus each containstwo identical square circuit patterns, each side of which has a lengthL. Shot areas as exposure units which are to be exposed by the secondexposure apparatus each contains three identical rectangular circuitpatterns in which one side has a length L, and the other side has alength 3L/2.

FIG. 9(a) shows a shot array on a wafer 20 loaded on a wafer stage (notshown) of the scanning type second exposure apparatus. In FIG. 9(a),square shot areas SA₁, SA₂, SA₃, . . . , SA₆₆, each side of which has alength L, are arranged at a predetermined pitch in each of thedirections X and Y. The shot areas SA₁, SA₂, . . . each has twoidentical circuit patterns 26A and 26B formed to lie side-by-side in adirection substantially parallel to the direction Y by the firstexposure apparatus, a developer, etc. In FIG. 9(a), X- and Y-axesrepresent a stage coordinate system of the second exposure apparatus.Further, the shot areas SA₁, SA₂, . . . have been each formed with twowafer marks MYA₁ and MYB₁ for the Y-axis and two wafer marks MXA₁ andMXB₁ for the X-axis, which are detectable by an LSA type detectionmethod. In FIG. 9(a), only the four wafer marks MYA₁, MYB₁, MXA₁ andMXB₁ in the shot area SA₁ are shown for the sake of simplicity.

In this example also, alignment is effected by the EGA method in thesame way as in the first example. In this example, however, each shotarea contains four one-dimensional wafer marks, and therefore, twoin-shot transformation parameters can be obtained in addition to theabove-described six transformation parameters (i.e. scaling parametersRx and Ry, shot array rotation θ, shot array perpendicularity error W,and offsets Ox and Oy). Accordingly, in this example, a shot rotation(chip rotation) θ [rad] and a shot perpendicularity error W [rad] areobtained as in-shot transformation parameters. It should be noted thatshot magnifications rx and ry can also be obtained by disposing twoother one-dimensional wafer marks in each shot area. However, thisexample is not particularly related to the determination of shotmagnifications rx and ry; therefore, wafer marks for them are notprovided in this example. A method in which EGA alignment is effected byusing three or more one-dimensional wafer marks or two or moretwo-dimensional wafer marks, which are disposed in each shot area, asdescribed above, is also known as “in-shot multipoint EGA alignmentmethod”.

More specifically, in this example, a predetermined number N (N is aninteger of 3 or more) of shot areas are selected from among shot areasSA₁ to SA₆₆ on a wafer 20 as sample shots S₁ to S₉ (in this case, N=9),and coordinate values in the stage coordinate system (X,Y) of two pairsof wafer marks attached to each of the sample shots S₁ to S₉ aremeasured by using an LSA type alignment system. For example, a meanvalue of the X coordinates of the two X-axis wafer marks of each sampleshot and a mean value of the Y coordinates of the two Y-axis wafer marksof the sample shot are regarded as array coordinates of the center ofthe sample shot, thereby obtaining parameters (i.e. scaling parametersRx and Ry, rotation θ, perpendicularity error W, and offsets Ox and Oy)for transformation from the sample coordinate system (x,y) into thestage coordinate system (X,Y) in the same way as in the first example.

Further, in this example, a shot rotation θ, which is a rotation angleδf the x-axis in a shot, is calculated on the basis of a mean value ofY-coordinate differences between the pairs of Y-axis wafer marks of thesample shots, for example, and a rotation angle θ_(y) of the y-axis in ashot is calculated on the basis of a mean value of X-coordinatedifferences between the pairs of X-axis wafer marks. The shot rotation θis subtracted from the rotation angle θ_(y) of the y-axis to obtain anangle w, which is determined to be a shot perpendicularity error.

It is assumed in this example that, as a result of the alignment, asshown in FIG. 9(a), the shot array rotation θ, the shot arrayperpendicularity error W and the shot perpendicularity error w havebecome capable of being regarded as zero, but the shot rotation θ hasbecome a predetermined value other than zero. On such a shot array,overlay exposure is effected by the scanning second exposure apparatushaving a shot area SC, as shown in FIG. 9(b), which has a size (3L/2)that is sufficiently large to contain three circuit patterns in thedirection Y as a scanning direction, and which has a width L in thedirection X. The method of alignment effected to carry out the overlayexposure will be explained below.

FIG. 10(a) shows a part of the shot array on the wafer 20 shown in FIG.9(a). In FIG. 10(a), square shot areas SA₁, SA₂ and SA₃, each side ofwhich has a length L, have been rotated counterclockwise through anangle corresponding to the shot rotation θ. It is assumed that circuitpattern images for two shot areas SC₁ and SC₂, each having the same sizeas the shot area SC shown in FIG. 9(b), are overlaid on the shot areasSA₁ to SA₃ by using the second exposure apparatus.

In this case, it is conceivable to effect alignment such that, as shownby the chain double-dashed lines in FIG. 10(a), reference points 31 aand 31 b, which are at the centers of two arrays of three circuitpatterns, coincide with the centers of the second-layer shot areas SC₁and SC₂, respectively, on an imaginary straight line 31 passing throughthe centers of the first-layer shot areas SA₁, SA₂ and SA₃ in parallelto the Y-axis. In this case, if the scanning direction in the secondexposure apparatus is restricted to the directions +Y and −Y, it isnecessary to rotate the wafer or the reticle clockwise through the angleθ, for example. Further, if the wafer is rotated, it is necessary tocorrect the array coordinates of each of the first-layer shot areas. Inthe following description, the scanning direction is assumed to be adirection intersecting the Y-axis at the angle θ.

With this alignment method, however, X-direction overlay errors a and b,which are given by the following equation (10), arise between thesecond-layer shot area SC₁ and the first-layer shot areas SA₁ and SA₂,respectively, in the same way as in a case where there is a shot arrayperpendicularity error as shown in FIG. 6(a): $\begin{matrix}{a = {{( {1/4} ){L \cdot \theta}\quad b} = {( {3/4} ){L \cdot \theta}}}} & (10)\end{matrix}$

Accordingly, it will be understood that the alignment method as shown inFIG. 10(a) causes a large overlay error b to arise particularly at thesecond shot area SA₂. In order to reduce the overlay error, in thisexample, the center positions of the second-layer shot areas SC₁ and SC₂are shifted by a predetermined distance from the reference points 31 aand 31 b in a direction perpendicular to the scanning direction.

FIG. 10(b) shows an alignment method carried out in this example. InFIG. 10(b), the center positions of the second-layer shot areas SC₁ andSC₂ have been shifted by −d and +d relative to the reference points 31 aand 31 b in a direction intersecting the X-axis at the angle θ in theclockwise direction. As a result, there is a uniform overlay error c inthe direction X between the first-layer shot areas SA₁ to SA₃ and thesecond-layer shot areas SC₁ and SC₂. The distance d and the overlayerror c are given by the following equation (11): $\begin{matrix}{d = {{( {1/4} ){L \cdot \theta}\quad c} = {( {1/2} ){L \cdot \theta}}}} & (11)\end{matrix}$

As a result, the overlay error c given by Eq. (11) is (½)L·θ in contrastto the overlay error b given by Eq. (10). Accordingly, it will beunderstood that the alignment method according to this example enablesthe maximum value of the overlay error to reduce to (½)L·θ, and thus theoverlay error reduces as a whole.

In the third example of the first embodiment, exposure may be carriedout by using the first exposure apparatus over shot areas exposed-byusing the scanning type second exposure apparatus in reverse relation tothe above-described exposure operation. One example of such an exposureoperation will be explained below with reference to FIG. 10(c).

FIG. 10(c) shows the first-layer shot areas SC₁ and SC₂ on the waferwhich have been formed with circuit patterns by using the scanningsecond exposure apparatus. In a case where, in FIG. 10(c), a reticlepattern image is to be formed for each of the shot areas SA₁, SA₂ andSA₃ over the shot areas SC₁ and SC₂ by using the first exposureapparatus, for the first and third shot areas SA₁ and SA₃, it is onlynecessary to align them with the first-layer shot areas SC₁ and SC₂,respectively, in the direction X. For the second shot area SA₂, it isonly necessary to align its position in a direction perpendicular to thescanning direction with an intermediate position between the first-layershot areas SC₁ and SC₂. Consequently, the overlay error between thefirst and second layers is b only at the shot area SA₂.

Further, for the second shot area SA₂, exposure may be effected for theupper and lower halves separately in the same way as in the methoddescribed with reference to FIGS. 8(b) and 8(c). By doing so, theoverlay error can be reduced to zero.

Although in the third example, only the shot rotation θ is corrected, itshould be noted that alignment may be effected as follows: A mean valueof the shot rotation θ obtained by the in-shot multipoint EGA method andthe shot perpendicularity error w, i.e. (θ+w)/2, is regarded as pseudoshot rotation, and alignment is effected on the basis of the pseudo shotrotation.

Further, although in the third example the shot rotation θ is obtainedby measuring the positions of three or more wafer marks in each sampleshot, the shot rotation θ may be obtained by using numerical valuespreviously obtained by test printing using the first exposure apparatus.In this case, in each sample shot the ordinary EGA type alignment iseffected by measuring the positions of a pair of wafer marks as in theconventional practice, and the shot rotation θ alone is obtained byusing the input numerical values.

Although in the above-described embodiment two steppers or a combinationof a stepper and a step-and-scan type projection exposure apparatus isused, it should be noted that, for example, two step-and-scan typeprojection exposure apparatuses may be used as two exposure apparatuseshaving respective exposure fields of different sizes.

The exposure method according to the first embodiment of the presentinvention provides the following advantages. In a case where exposure iscarried out by the mix-and-match method using two exposure apparatuseshaving respective exposure fields of different sizes, a perpendicularityerror in the shot area array on the preceding layer or a mean value ofrotation angles of the shot areas is detected, and the shot areas of thesubsequent layer are rotated according to the result of the detection.Accordingly, the overlay error between the two layers can be favorablyreduced.

Further, in the exposure method according to the first embodiment of thepresent invention, when exposure is to be carried out by themix-and-match method using two exposure apparatuses having respectiveexposure fields of different sizes, a perpendicularity error in the shotarea array on the preceding layer or a mean value of rotation angles ofthe shot areas is detected, and exposure is carried out with the shotareas of the subsequent layer shifted in a direction perpendicular tothe scanning direction of the second exposure apparatus according to theresult of the detection. Accordingly, the overlay error between the twolayers can be favorably reduced.

In this case, if the shot areas of the subsequent layer are rotated inaddition to the shifting of the shot areas, the overlay error may befurther reduced.

Next, a first example of a second embodiment of the exposure methodaccording to the present invention will be described with reference toFIGS. 11 to 14(c). Two exposure apparatuses used in this example are aone-shot exposure type projection exposure apparatus (stepper) with ademagnification ratio of 5:1 and a step-and-scan type projectionexposure apparatus with a demagnification ratio of 4:1. In this example,two chip patterns ale formed in each shot area exposed by the formerprojection exposure apparatus (i.e. a two-chip reticle is used), andthree chip patterns are formed in each shot area scan-exposed by thelatter projection exposure apparatus (i.e. a three-chip reticle isused). It should be noted that constituent elements in the secondembodiment which are similar to those in the first embodiment aredenoted by the same reference characters.

FIG. 11 shows an exposure system used in this example. In FIG. 11, astepper 1A, which is a one-shot exposure type projection exposureapparatus, and a step-and-scan type projection exposure apparatus(hereinafter referred to as “scanning exposure apparatus”) 1B areinstalled. In this example, the stepper 1A is a high-resolution exposureapparatus, while the scanning exposure apparatus 1B is a low-resolutionexposure apparatus. The stepper 1A is used to carry out exposure for acritical layer, which requires high resolution, on a wafer, and thescanning exposure apparatus 1B is used to carry out exposure for amiddle layer, which does not require high resolution, on the wafer.However, the stepper 1A may be a low-resolution exposure apparatus orthe scanning exposure apparatus 1B may be a high-resolution exposureapparatus according to the kind of semiconductor device to be produced.

First, in the stepper 1A, a pattern area 42A on a reticle RA isilluminated by exposure light from an illumination optical system (notshown), and an image of a pattern formed in the pattern area 42A isformed on a rectangular exposure field 44A on a wafer 20 as a projectedimage reduced to ⅕ by a projection optical system 3A. A Z1-axis is takenin a direction parallel to an optical axis of the projection opticalsystem 3A, and two axes of an orthogonal coordinate system set in aplane perpendicular to the Z1-axis are defined as an X1-axis and aY1-axis, respectively. The pattern area 42A on the reticle RA is dividedinto partial pattern areas 112A and 112B of the same size in apredetermined direction (in FIG. 11, in the direction Y1). The partialpattern areas 112A and 112B each has original drawing patterns ofcircuit patterns and alignment marks arranged according to the samelayout.

The wafer 20 is held on a wafer stage 5A. The wafer stage SA comprises aZ-stage for moving the wafer 20 in the direction Z1 to set an exposuresurface of the wafer 20, which is to be exposed, at the best focusposition, and an XY-stage for positioning the wafer 20 in both thedirections of the X1- and Y1-axes. A pair of moving mirrors 6A and 8Awhich are perpendicular to each other are fixed on the wafer stage 5A.The coordinate in the direction X1 of the wafer stage 5A is measured bya combination of the moving mirror 6A and a laser interferometer 7Awhich is installed outside the wafer stage 5A. The coordinate in thedirection Y1 of the wafer stage 5A is measured by a combination of themoving mirror 8A and a laser interferometer 9A which is installedoutside the wafer stage 5A. The coordinates measured by the laserinterferometers 7A and 9A are supplied to a controller 10A whichcontrols operations of the whole apparatus. The controller 10A drivesthe wafer stage 5A to step in both the directions X1 and Y1 throughdrive units (not shown), thereby positioning the wafer 20. In this case,the stepping drive of the wafer 20 is effected according to an array ofshot areas (i.e. unit areas to each of which a pattern image of thepattern area 42A is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a critical layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10A. It is assumed that a predeterminedperpendicularity error W remains in a coordinate system (i.e. stagecoordinate system) (X1,Y1) which defines the travel position of thewafer stage 5A of the stepper 1A in this example.

Further, the stepper 1A in this example is provided with an off-axisimaging type (FIA type) alignment system 11A. The alignment system 11Aimages an alignment mark (wafer mark) on the wafer 20 and processes animaging signal thus obtained to detect X1 and Y1 coordinates of thewafer mark. The detected coordinates are supplied to the controller 10A.

It should be noted that, as the alignment system 11A, it is alsopossible to use a TTR (Through-The-Reticle) type alignment system or aTTL (Through-The-Lens) type alignment system in which the position of amark is detected through the projection optical system 3A. As a markdetecting method, it is also possible to use a laser step alignment(LSA) method in which a slit-shaped laser beam and a mark are scannedrelative to each other, or a so-called two-beam interference method (LIAmethod) in which two light beams are applied to a diffractiongrating-shaped mark, and the position of the mark is detected from asignal obtained from interference between a pair of diffracted lightbeams generated in parallel from the illuminated mark.

Next, in the scanning exposure apparatus 1B in this example, a part of apattern area 42B on a reticle RB is illuminated by exposure light froman illumination optical system (not shown), and an image of a part ofthe reticle pattern is formed in a slit-shaped exposure area 144 on awafer 20, which is held on a wafer stage 5B, as a projected imagereduced to ¼ by a projection optical system 3B. Here, a Z2-axis is takenin a direction parallel to an optical axis of the projection opticalsystem 3B, and two axes of an orthogonal coordinate system set in aplane perpendicular to the Z2-axis are defined as an X2-axis and aY2-axis, respectively. Under these circumstances, the reticle RB isscanned in the direction −Y2 (or +Y2), and the wafer 20 is scanned inthe direction +Y2 (or −Y2) in synchronism with the scanning of thereticle RS, thereby sequentially projecting an image of the patternformed in the pattern area 42B of the reticle RB onto the exposure field44B on the wafer 20.

The pattern area 42B of the reticle RB is divided into three partialpattern areas 13A to 13C of the same size in the direction Y2, which isthe scanning direction. The size of the exposure field 44B is such thatits dimension in the scanning direction is 3/2 times as large as thedimension of the exposure field 44A of the stepper 1A, and the exposurefield 44B is equal in size (1:1) to the exposure field 44A in thenon-scanning direction. That is, the exposure field 44B is longer thanthe exposure field 44A in the direction Y2.

The position of a reticle stage (not shown) for scanning the reticle RBof the scanning exposure apparatus 1B is measured by a laserinterferometer (not shown). The X2 coordinate of the wafer stage 5B ismeasured by a combination of a moving mirror 6B and a laserinterferometer 7B, and the Y2 coordinate of the wafer stage 5B ismeasured by a combination of a moving mirror 8B and a laserinterferometer 9B. The measured coordinates of the wafer stage 52 aresupplied to a controller 10B. In this example, the X2- and Y2-axes areassumed to be perpendicular to each other. The controller 10B controlssynchronous drive of the reticle stage (not shown) and the wafer stage5B. Scanning exposure of the wafer stage 5B is effected according to ashot map for a middle layer set on an exposure surface of the wafer 20,which is to be exposed. The shot map is generated by a map generatingunit which comprises a computer in the controller 10B.

In this case, the map generating unit in the controller 10A and the mapgenerating unit in the controller 10B have the function of supplyingshot map information prepared thereby to each other. When exposure for amiddle layer is to be carried out over a critical layer, for example;shot map information for the critical layer prepared by the mapgenerating unit in the controller 10A of the stepper 1A is transmittedto the other controller 10B. The map generating unit in the controller10B generates a shot map for the middle layer on the basis of thesupplied shot map information. Conversely, when exposure for a criticallayer is to be carried out over a middle layer, shot map information forthe middle layer prepared in the controller 10B is supplied to thecontroller 10A.

The scanning exposure apparatus 1B also has an off-axis imaging type(FIA type) alignment system 11B provided at a side surface of theprojection optical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark on the wafer 20.

Next, one example of an exposure operation which is performed in thisexample when exposure for a first-layer pattern is first effected byusing the stepper 1A and then exposure for a second-layer pattern iseffected by using the scanning exposure apparatus 1B will be explainedfor each of the first and second processing steps.

First, the first step will be explained.

In the first step, as shown in FIG. 12(a), the reticle RA is fixed onthe reticle stage (not shown) of the stepper 1A, shown in FIG. 11, suchthat the reticle RA is rotated through 90° from its ordinary position.As a result, the two partial pattern areas 112A and 112B in the patternarea 42A of the reticle RA lie side-by-side in the direction X1. Next,as shown in FIG. 12(b), the wafer 20 coated with a photoresist is fixedon the wafer stage 5A of the stepper 1A, shown in FIG. 11, such that thewafer 20 is rotated through 90° from its ordinary position. As a result,the wafer 20 is placed such that the cut portion (orientation flat) ofthe outer periphery of the wafer 20 faces in the direction +X1. Althoughin FIG. 12(b) the mutual origin of the X1 and Y1-axes is set at thecenter of the wafer 20, in FIG. 12(a) the origin of the two axes is setoutside the reticle RA for the sake of explanation. Further, in FIG.12(a), the reticle RA is shown in the size of an image thereof asprojected on the wafer 20. In this example, as shown in FIG. 12(b), theY1-axis has rotated through an angle W clockwise relative to animaginary axis Y1* perpendicularly intersecting the X1-axis; the angle Wis a perpendicularity error.

Next, a pattern image of the reticle RA is sequentially projected ontoshot areas 121A, 121B, . . . , 121I, which are obtained by dividing afirst-layer exposure area on the wafer 20 at a predetermined pitch ineach of the directions X1 and Y1, by the step-and-repeat method usingthe stepper 1A. For example, the first-layer shot areas 121A to 121I arearranged in an array of 3 columns in the direction X1 and 3 rows in thedirection Y1. In this case, there is the perpendicularity error Wbetween the X1- and Y1-axes; therefore, the array of shot areas 121A to1211 also has the perpendicularity error W.

However, in this example, exposure has been effected with the reticle RAand the wafer 20 each rotated through 90°. Therefore, in FIG. 12(b), thefirst shot area 121A on the wafer 20 has two partial shot areas 122A and122B divided in the direction X1. The partial shot areas 122A and 122Bhave been exposed to pattern images which are identical with each other.The same is the case with the other shot areas 121B to 121I. When edgesof the shot areas 121A to 121C in the first row which are parallel tothe array direction of the partial shot areas 122A and 122B areconnected together, a straight line 125, which has no irregularity, isobtained.

Thereafter, the wafer 20 is subjected to development, thereby allowingthe circuit pattern images and alignment mark images in each shot areato appear as circuit patterns and wafer marks, which compriserecess-and-projection patterns. In this example, the first partial shotarea 122A in the first shot area 121A is formed with an X-axis wafermark 123X and a Y-axis wafer mark 123Y, which comprise line-and-spacepatterns, respectively. The second partial shot area 122B is also formedwith an X-axis wafer mark 124X and a Y-axis wafer mark 124Y. The wafermarks 123X, 123Y, 124X and 124Y are marks which are detectable with animaging type alignment sensor. It should be noted that the arrangementof wafer marks is not necessarily limited to the example shown in FIG.12(b). For example, the arrangement of wafer marks may be such that apair of wafer marks are disposed in each of the shot areas 121A to 1211.It is also possible to dispose more than one pair of wafer marks in eachof the shot areas 121A to 1211. Further, two-dimensional marks may beused as wafer marks.

Next, the second step will be explained.

A photoresist is coated over the wafer 20 having the circuit patternsand wafer marks formed thereon in the first step. The photoresist-coatedwafer 20 is fixed at the ordinary rotation angle on the wafer stage 5Bof the scanning exposure apparatus 1B, shown in FIG. 11. Thus, as shownin FIG. 13, the wafer 20 is disposed such that its cut portion faces inthe direction −Y2. As shown in FIG. 11, the reticle RB for the secondlayer is also set at the ordinary rotation angle, that is, at an angleat which the partial pattern areas 113A to 113C are arranged in thedirection Y2.

At this time, the first-layer shot array data is supplied from thecontroller 10A of the stepper 1A to the controller 10B of the scanningexposure apparatus 1B. The controller 10B determines a second-layer shotarray on the basis of the supplied shot array data, together withalignment data (described later).

Thereafter, the wafer 20, which is an object to be exposed, is subjectedto alignment by the EGA method in the scanning exposure apparatus 1B.

FIG. 13 shows the wafer 20 as an object to be exposed. In FIG. 13, theorigin of the stage coordinate system (X2,Y2) of the scanning exposureapparatus 1B is set at the center of the wafer 20 for the sake ofconvenience. The origin of the stage coordinate system (X1,Y1) of thestepper. 1A used to expose the first layer is also shown to becoincident with the center of the wafer 20. In this case, because thewafer 20 is set at the ordinary rotation angle, the two partial shotareas 122A and 122B in the shot area 121A, for example, are arranged inthe direction Y2, and in each of the other shot areas 121B to 121I, thetwo partial shot areas are also arranged in the direction Y2. To effectEGA type alignment, three or more shot areas are selected as sampleshots from among the nine shot areas 121A to 121I on the wafer 20, andthe coordinates in the stage coordinate system (X2,Y2) of the wafermarks in the sample shots are measured by using the alignment system11B, shown in FIG. 11. When the shot area 121A, for example, is selectedas a sample shot, the coordinates of a pair of wafer marks 123X and 123Yin the first partial shot area 122A of the shot area 121A are measured,and for each of the other sample shots also, the coordinates of a pairof wafer marks are measured.

Next, the measured values of the coordinates of the wafer marks in thesample shots and the design array coordinates of these wafer marks arestatistically processed to determine values of EGA parameters includinga rotation (wafer rotation) θ₁ of the first-layer shot array, aperpendicularity error W₁ of the first-layer shot array, an offset Ox₁in the direction X2, and an offset Oy₁ in the direction Y2. In thisexample, the wafer 20 was rotated through 90° at the time of exposurefor the first layer, and the X2- and Y2-axes of the second layer areassumed to be perpendicular to each other. Therefore, the rotation θ₁ isan angle between the Y1-axis of the first-layer shot array and theX2-axis of the second-layer stage coordinate system, and theperpendicularity error W₁ is equal to an angle obtained by subtractingπ/2 (90°) from the angle between the Y1-axis and the axis (−X1-axis)which is in inverse relation to the X1-axis, that is, theperpendicularity error W in FIG. 12(b).

After the EGA parameters have been obtained as described above, thescanning exposure apparatus 1B, shown in FIG. 11, sets the rotationangle of the wafer 20 such that the Y1-axis of the first-layer shotarray is rotated clockwise relative to the X2-axis through the sameangle as the perpendicularity error W₁ (i.e. W). This means that anoffset of the same angle as the perpendicularity error W₁ is added tothe desired value of the shot array rotation (wafer rotation). As aresult, a straight line 125 which connects together the right-hand edgesof the shot areas 121A to 121C in the first column, which is parallel tothe array direction of the partial shot areas 122A and 122B of the shotarea 121A, for example, becomes parallel to the direction Y2, which isthe scanning direction of the scanning exposure apparatus 1B. In thisstate, a shot array of the second layer is determined by taking intoconsideration the offsets Ox₁ and 0y₁ in the EGA parameters.

FIG. 14(a) shows a second-layer shot array set over the first layer asdescribed above. In FIG. 14(a), for example, second-layer shot areas126A and 126B are set over the first-layer shot areas 121A to 121C;similarly, other second-layer shot areas 126C to 126F are set. In thiscase, the shot area 126A, for example, is divided into three partialshot areas 127A to 127C in the direction Y2. The partial shot areas 127Ato 127C are respectively exposed to images of patterns in partialpattern areas 113C to 113A of the reticle RB, shown in FIG. 14(b). Thepartial shot areas 127A to 127C in the second-layer shot area 126A eachhas the same size as the size of each of the partial shot areas 122A and122B in the first-layer shot area 121A, shown in FIG. 14(c). The othersecond-layer shot areas 126B to 126F also each has the sameconfiguration as that of the second-layer shot area 126A. The shot areas126A to 126F determined in this way are each exposed to a pattern imageof the reticle RB by the scanning exposure method. Thereafter,development and other processing are carried out, thereby allowingpatterns to appear in the second-layer shot areas.

In this example, as shown in FIG. 14(a), the straight line 125connecting the right-hand edges of the first-layer shot areas 121A to121C is parallel to the direction Y2, which is the scanning directionfor the second layer; therefore, the second-layer shot areas 126A and126B are overlaid on the first-layer shot areas 121A to 121Csubstantially perfectly in both the directions X2 and Y2. Thus, theeffect of the perpendicularity error in the first-layer shot array iseliminated.

Although in the above-described example the X2- and Y2-axes of the stagecoordinate system in the scanning exposure apparatus 1B are assumed tobe perpendicular to each other as shown in FIG. 13, it should be notedthat the X2- and Y2-axes do not necessarily need to be perpendicular toeach other. In a case where the X2- and Y2-axes are not perpendicular toeach other, the wafer 20 should be rotated such that the X1-axis of thefirst-layer shot array is parallel to the direction Y2, which is thescanning direction.

Next, a second example of the second embodiment of the present inventionwill be described with reference to FIGS. 15 and 16.

In this example also, the two projection exposure apparatuses (i.e.stepper 1A and scanning exposure apparatus 1B) shown in FIG. 11 areused. First, exposure is carried out with respect to the wafer 20 in thestepper 1A in a state where the reticle RA and the wafer 20 have beeneach rotated through 90° relative to their ordinary positions, as shownin FIGS. 12(a) and 12(b). Next, the wafer 20 is restored to the ordinaryrotation angle in the scanning exposure apparatus 1B, as shown in FIG.13, and then, measurement for alignment is carried out. Up to thispoint, the second example is approximately the same as the firstexample. In this example, however, the rotation angle of each sampleshot is also measured during the measurement of coordinates of the wafermarks in the sample shots. When the shot area 121A, for example, is asample shot, the rotation angle thereof is measured as follows: Forexample; the coordinates of a pair of wafer marks 123X and 123Y aremeasured, and at the same time, the X2 coordinate of another X-axiswafer mark 124X is measured. A difference between the X2 coordinates ofthe wafer marks 123X and 124X is divided by an approximate value of thedistance in the direction Y2 between the two wafer marks 123X and 124Xto obtain a rotation angle of the shot area 121A. Similarly, rotationangles of the other sample shots are obtained, and a mean value of theobtained rotation angles is defined as a shot rotation θ_(s). Analignment method in which the coordinates of the number of wafer markswhich exceeds two (one in the case of two-dimensional wafer marks) aremeasured in each sample shot is called “in-shot multipoint EGA alignmentmethod”.

Next, the rotation angle of the wafer 20 is set without adding an offsetto the desired value of the shot array rotation (wafer rotation) in thisexample.

FIG. 15 shows the wafer 20 having a rotation angle set as describedabove on the wafer stage 5B in the scanning exposure apparatus 1B, shownin FIG. 11. In FIG. 15, a Y1-axis which indicates one array direction ofthe first-layer shot array is set parallel to the X2-axis of the stagecoordinate system in the scanning exposure apparatus 1B. As a result,the right-hand edges of the shot areas 121A, 121B and 121C, which areparallel to the array direction of the two partial shot areas in thefirst-layer shot area 121A, for example, are slanted at the same angleas the perpendicularity error W with respect to the direction Y2, whichis the scanning direction. The perpendicularity error W is a valueobtained by subtracting the shot rotation θ_(s), obtained by the in-shotmultipoint EGA method, from the shot array rotation θ_(t) obtained inthe state shown in FIG. 13. If exposure is simply carried out by thescanning exposure method with respect to the wafer 20 which is in thestate shown in FIG. 15, second-layer shot areas would become shot areas126A to 126F having edges parallel to the directions X2 and Y2, as shownby the chain double-dashed lines, resulting in an overlay error betweenthe first- and second-layer shot areas.

In order to avoid the above problem, in this example, exposure iscarried out with each of the second-layer shot areas 126A to 126Frotated counterclockwise through an angle equal to the perpendicularityerror W relative to the wafer 20. More specifically, in a case where thescanning direction can be finely adjusted during the scanning exposurein the scanning exposure apparatus 1B, the reticle RB is rotatedcounterclockwise through an angle equal to the perpendicularity error Win FIG. 11, and thereafter, the reticle RB is scanned in a directionrotated from the direction Y2 by the perpendicularity error W, and thewafer 20, shown in FIG. 15, is scanned in a direction parallel to thescanning direction of the reticle RB in synchronism with the scanning ofthe reticle RB. To finely adjust the scanning direction of the reticleRB in this way, the reticle RB should be gradually shifted in thedirection X2 according to the scanning position by using, for example, amechanism for finely adjusting the position of the reticle RB. As aresult, the second-layer shot areas 126A to 126F are each rotatedcounterclockwise by the perpendicularity error W, as shown by thecontinuous lines in FIG. 16, and thus the overlay error between thefirst- and second-layer shot areas is minimized.

It should be noted that, when the exposure apparatus that effectsexposure for the second layer is a one-shot exposure type projectionexposure apparatus (e.g. stepper), the rotation angle (shot rotation) ofeach of the shot areas 126A to 126F can be corrected, as shown in FIG.16, simply by rotating the reticle RB.

Next, a third example of the second embodiment of the present inventionwill be described with reference to FIGS. 17(a), 17(b) and 17(c).

In this example also, the two projection exposure apparatuses (i.e.stepper 1A and scanning exposure apparatus 1B) shown in FIG. 11 areused. First, as shown in FIGS. 12(a) and 12(b), exposure is carried outwith respect to the wafer W in the stepper 1A in a state where thereticle RA and the wafer 20 have been each rotated through 90° relativeto their ordinary positions. Up to this point, the third example is thesame as the first example. In this example, however, the subsequentexposure for the second layer is carried out by the scanning exposureapparatus 1B, shown in FIG. 11, with the reticle RB and the wafer 20left rotated through 90° from their ordinary positions. The scanningexposure apparatus 1B in this example effects scanning exposure in adirection parallel to the direction X2. For this purpose, an exposureapparatus in which the scanning direction is the direction X2 should beused as the scanning exposure apparatus 1B. Alternatively, an exposureapparatus in which the scanning direction can be switched to either ofthe directions X2 and Y2 should be used as the scanning exposureapparatus 1B. The following is a description of the exposure method forthe second layer in this example.

FIG. 17(a) shows the wafer 20 placed on the wafer stage 5B in thescanning exposure apparatus 1B, shown in FIG. 11, after the completionof exposure and development for the first layer. In FIG. 17(a), theX1-axis which indicates one array direction of first-layer shot areas121A, 121B, . . . , 121I is set approximately parallel to the X2-axis ofthe stage coordinate system in the scanning exposure apparatus 1B. Inthis case, at the time of exposure for the first layer, as shown in FIG.17(c), the reticle RA is set such that the two partial pattern areas112A and 112B lie side-by-side in the direction X1. The first-layer shotarray has a perpendicularity error W.

In this example also, the wafer 20 shown in FIG. 17(a) is subjected toEGA alignment, thereby obtaining values of EGA parameters including ashot array rotation (wafer rotation) θ₂, a shot array perpendicularityerror W₂, an offset Ox₂ in the direction X2, and an offset Oy₂ in thedirection Y2. Thereafter, the rotation angle of the wafer 20 is set suchthat the X1-axis is accurately parallel to the X2-axis on the basis ofthe rotation θ₂. Then, as shown in FIG. 17(b), the rotation angle of thereticle RB is set such that the array direction of the partial patternareas 113A to 113C is parallel to the direction X2. Then, the reticle RBis scanned in the direction +X2 (or −X2) by the scanning exposureapparatus 1B, and the wafer 20 is scanned in the direction −X2 (or +X2)by the scanning exposure apparatus 1B in synchronism with the scanningof the reticle RB, thereby sequentially transferring a pattern image ofthe reticle RB onto the second-layer shot areas 126A, 126B, . . . ,126F, shown in FIG. 17(a), by the scanning exposure method. As a result,the second-layer shot areas 126A and 126B are substantially perfectlyoverlaid on the first-layer shot areas 121A, 121B and 121C, and thus theeffect of the perpendicularity error W of the first layer is eliminated.

Although in the above-described second embodiment exposure is firstcarried out by the stepper 1A having a small exposure field, andthereafter, exposure is carried out by the scanning exposure apparatus1B having a large exposure field, it should be noted that the presentinvention is also applicable to an exposure process in which exposure iscarried out by the stepper 1A having a small exposure field afterexposure has been carried out by the scanning exposure apparatus 1Bhaving a large exposure field in reverse relation to the above. In thelatter case also, the effect of the perpendicularity error of the firstlayer can be reduced. Further, although in the above-described secondembodiment the stepper 1A and the scanning exposure apparatus 1B areused as a combination of two exposure apparatuses, it should be notedthat both the two exposure apparatuses may be steppers. Alternatively,both the two exposure apparatuses may be scanning exposure apparatuses.

According to the second embodiment, when a first mask pattern is to betransferred onto a photosensitive substrate by using the first exposureapparatus, the array of a plurality of shot areas on the photosensitivesubstrate to each of which the first mask pattern is to be transferredby exposure is set in a direction corresponding to a direction in whichthe exposure field of the first exposure apparatus is different inlength from the exposure field of the second exposure apparatus (i.e.the second exposure field). Therefore, a plurality of shot areas of afirst layer can be arranged in the form of a straight line in thedirection in which the exposure fields of the first and second exposureapparatuses differ in length from each other. Accordingly, even if thefirst-layer shot array has a perpendicularity error, an overlay errorbetween the first and second layers can be reduced by overlaying thesecond-layer shot areas on the first-layer shot areas along thedirection in which the two exposure fields are different in length fromeach other. Thus, it is possible to reduce an overlay error betweendifferent layers in a case where exposure is carried out by themix-and-match method using a plurality of exposure apparatuses havingrespective exposure fields (shot areas) of different sizes because theyare different from each other in the length in a predetermined directionon a photosensitive substrate.

In a case where the first mask pattern is transferred onto thephotosensitive substrate by using the first exposure apparatus in astate where the photosensitive substrate and the first mask pattern havepreviously been rotated through 90° from their ordinary positions, theeffect of a perpendicularity error of the first-layer shot array can bereadily eliminated without providing a special mechanism on the twoexposure apparatuses, particularly when the first exposure apparatus isa one-shot exposure type exposure apparatus (e.g. stepper).

In a case where the second exposure apparatus is a scanning exposuretype exposure apparatus, and the above-described predetermined direction(i.e. direction in which the two exposure fields differ in length fromeach other) is the scanning direction, the exposure field (i.e. secondexposure field) of the second exposure apparatus is likely to lengthenin the predetermined direction in particular. Accordingly, the presentinvention is particularly useful in such a case.

Although in the second embodiment the one-shot exposure type exposureapparatus is used first and then the scanning exposure type exposureapparatus is used, these two exposure apparatuses may be used in thereverse order. That is, the scanning exposure type exposure apparatusmay be used first.

Next, a third embodiment of the exposure method according to the presentinvention will be described with reference to FIGS. 18 to 23(b). In thisembodiment, a projection exposure apparatus (stepper) in which a reducedimage of a pattern formed on a reticle is projected onto each shot areaon a wafer by the step-and-repeat method is used as each of two exposureapparatuses. It should be noted that constituent elements in thisembodiment which are similar to those in the first and secondembodiments are denoted by the same reference characters, and thatarrangements similar to those in the first and second embodiments willbe briefly explained in the following description.

FIG. 18 shows an exposure system used in this embodiment. In theillustrated exposure system are installed a stepper 1A having a smallexposure field (hereinafter referred to as “fine stepper”) and a stepper1B having a large exposure field (hereinafter referred to as “middlestepper”). In this embodiment, the fine stepper 1A is a high-resolutionexposure apparatus, and the middle stepper 1B is a low-resolutionexposure apparatus. The fine stepper 1A is used to carry out exposurefor a critical layer on a wafer, and the middle stepper 11 is used tocarry out exposure for a middle layer on the wafer. However, the finestepper 1A may be a low-resolution exposure apparatus or the middlestepper 1B may be a high-resolution exposure apparatus according to thekind of semiconductor device to be produced.

First, in the stepper 1A, a pattern area 52A on a reticle RA isilluminated by exposure light from an illumination optical system (notshown), and an image of the original drawing patterns of overlayaccuracy measuring marks (vernier marks), which have been written in thepattern area 52A according to a predetermined layout, is projected ontoa rectangular exposure field 54A on a wafer 20 as an image reduced to ⅕by a projection optical system 3A. A Z1-axis is taken in a directionparallel to an optical axis of the projection optical system 3A, and twoaxes of an orthogonal coordinate system set in a plane perpendicular tothe Z1-axis are defined as an X1-axis and a Y1-axis, respectively. Thereticle RA has an alignment mark 217X for the X1-axis formed at an endof the pattern area 52A in the direction Y1 (e.g. within a maskingframe) and also has an alignment mark 217Y for the Y1-axis formed at anend of the pattern area 52A in the direction X1.

A wafer stage 5A comprises a Z-stage, an XY-stage, etc. The coordinatein the direction X1 of the wafer stage 5A is measured by a combinationof a moving mirror 6A and a laser interferometer 7A. The coordinate inthe direction Y1 of the wafer stage 5A is measured by a combination of amoving mirror 8A and a laser interferometer 9A. The coordinates measuredby the laser interferometers 7A and 9A are supplied to a controller 10Awhich controls operations of the whole apparatus. The controller 10Adrives the wafer stage 5A to step, thereby positioning the wafer 20. Inthis case, the stepping drive of the wafer 20 is effected according to ashot map for a critical layer. The shot map is generated by a mapgenerating unit which comprises a computer in the controller 10A.

An off-axis imaging type (FIA type) alignment system 11A images analignment mark (wafer mark) or overlay accuracy measuring vernier markon the wafer 20 and processes an imaging signal thus obtained to detectX1 and Y1 coordinates of the mark. The detected coordinates are suppliedto the controller 10A.

The middle stepper 1B in this embodiment has substantially the samearrangement as that of the fine stepper 1A. In the middle stepper 1B,however, an image of a pattern formed in a pattern area 52B of a reticleRB is projected onto a rectangular exposure field 54B on a wafer 20 heldon a wafer stage 5B as an image reduced to {fraction (1/2.5)} through aprojection optical system 3B. Accordingly, the size of the exposurefield 54B is double that of the exposure field 54A of the fine stepper1A in both lengthwise and breadthwise directions. A Z2-axis is taken ina direction parallel to an optical axis of the projection optical system3B, and two axes of an orthogonal coordinate system set in a planeperpendicular to the Z2-axis are defined as an X2-axis and a Y2-axis,respectively. The pattern area 52B of the reticle RB is divided into twocolumns in the direction X2 and two rows in the direction Y2 to formpartial pattern areas 218A to 218D. The partial pattern areas 218A to218D each has vernier mark original drawing patterns formed according tothe same layout.

The X2 coordinate of the wafer stage 5B in the middle stepper 1B ismeasured by a combination of a moving mirror 6B and a laserinterferometer 7B. The Y2 coordinate of the wafer stage 5B is measuredby a combination of a moving mirror 8B and a laser interferometer 9B.The measured coordinates of the wafer stage 5B are supplied to acontroller 106. The controller 10B controls stepping of the wafer stage5B. Stepping drive of the wafer stage 5B is effected according to anarray of shot areas (to each of which the pattern image of the patternarea 52B is to be projected by exposure) set on an exposure surface ofthe wafer 20, which is to be exposed, that is, a shot map for a middlelayer. The shot map is generated by a map generating unit whichcomprises a computer in the controller 10B.

In this case, the map generating unit in the controller 10A and the mapgenerating unit in the controller 10B have the function of supplyingshot map information prepared thereby to each other. When exposure for amiddle layer is to be carried out over a critical layer, for example,shot map information for the critical layer prepared by the mapgenerating unit in the controller 10A of the stepper 1A is transmittedfrom a communication unit in the controller 10A to a communication unitin the controller 10B. The map generating unit in the controller 10Bgenerates a shot map for the middle layer on the basis of the suppliedshot map information. Conversely, when exposure for a critical layer isto be carried out over a middle layer, shot map information for themiddle layer prepared by the map generating unit in the controller 10Bis supplied to the map generating unit in the controller 10A.

The middle stepper 1B also has an off-axis imaging type (FIA type)alignment system 11B provided at a side surface of the projectionoptical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark or vernier mark on the wafer 20.

Next, one example of an operation of correcting coordinatetransformation parameters for alignment when exposure of the pattern forthe middle layer is to be effected by the middle stepper 1B afterexposure of the pattern for the critical layer has been carried out bythe fine stepper 1A in this embodiment will be explained for each of thefirst to third processing steps. In this embodiment also, the EGA(Enhanced Global Alignment) method is used in which values of the sixcoordinate transformation parameters (scaling parameters Rx and Ry,rotation θ, perpendicularity W, and offsets Ox and Oy) in Eq. (1) aredetermined, and array coordinates of each shot area are calculated fromthe coordinate transformation parameters and design array coordinates.

First, the first step will be explained.

In the first step, an unexposed wafer 20 coated with a photoresist isplaced on the wafer stage 5A of the fine stepper 1A, shown in FIG. 18,and a reduced image of the pattern on the reticle RA is sequentiallytransferred by the step-and-repeat method onto a multiplicity of shotareas arrayed on the wafer 20 in units of the exposure field 54A. Thereticle RA has the original drawing patterns of a plurality of verniermarks formed according to a predetermined layout in addition to a pairof alignment marks. Thereafter, the wafer 20 is subjected todevelopment, thereby allowing the pair of alignment marks to appear aswafer marks comprising recess-and-projection patterns, and also allowingthe vernier mark original drawing patterns to appear as vernier markscomprising recess-and-projection patterns. The patterns obtained afterthe development can be regarded as critical layer patterns on the wafer20. However, it is also possible to carry out the following alignmentand measurement of an amount of positional displacement between twocorresponding vernier marks with these marks left in the form of latentimages without effecting development.

Next, the second step will be explained.

A photoresist is coated over the wafer 20 having the wafer and verniermarks formed in the first step, and the photoresist-coated wafer 20 isplaced on the wafer stage 5B of the middle stepper 1B, shown in FIG. 18.At this time, information concerning the critical layer shot map used inthe first step has been supplied from the controller 10A of the finestepper 1A to the controller 10B of the middle stepper 1B. Thus, thecontroller 10B can obtain design array coordinates of the critical layerwafer marks on the wafer 20.

FIG. 19(a) shows the wafer 20 placed on the wafer stage 5B. In FIG.19(a), the X2- and Y2-axes of the middle stepper 1B are shown as beingX- and Y-axes, respectively. In this case, the wafer 20 has been roughlyaligned by a pre-alignment mechanism (not shown), and the surface of thewafer 20 has been divided into M (M is an integer of 12 or more)critical layer shot areas SE1, SE2, . . . , SEM in two directions whichare approximately parallel to the directions X and Y, respectively. Inactual practice, a scribe line area of a predetermined width liesbetween shot areas SEm (m=1 to M); however, illustration of the scribeline area is omitted in FIG. 19(a). The width (pitch) in the direction Xof each shot area SEm, including the scribe line area, is d, and thewidth (pitch) in the direction Y is c. In this embodiment, each shotarea SEm is approximately square (d≈c).

FIG. 19(b) shows a shot area SEm as a typical example of the criticallayer shot areas. In FIG. 19(b), the shot area SEm has a wafer mark 221Xfor the X-axis formed at one end thereof, and also has a wafer mark 221Yfor the Y-axis formed at another end thereof. Further, the shot area SEmhas five vernier marks 222A to 222B which are distributed in a crossshape, and also has four vernier marks 223A to 223D which are formed atrespective positions near the four corners of the shot area SEm. Theoriginal drawing patterns of marks distributed as shown in FIG. 19(b)have been formed in the pattern area 52A of the reticle RA of the finestepper 1A, shown in FIG. 18.

It should be noted that the vernier marks 222A to 222E and 223A to 223Dused in this embodiment are two-dimensional box-in-box marks, which aredetected by an imaging detection method with the alignment system 11B,shown in FIG. 18. However, it is possible to use other kinds of mark asvernier marks, for example, marks each formed by a combination of twoone-dimensional line-and-space patterns which are crossed at rightangles. It is also possible to use the wafer marks 221X and 221Y asvernier marks. Further, marks which are detected, for example, by thelaser step alignment (LSA) method may also be used as vernier marks. Thedistribution of vernier marks is not necessarily limited to that shownin FIG. 19(b). That is, vernier marks used in this embodiment may bedistributed as desired.

Next, the controller 10B of the middle stepper 1B, shown in FIG. 18,effects alignment by the EGA method. Accordingly, the controller 10Bdrives the wafer stage 5B to move the field of view of the alignmentsystem 11B sequentially according to the critical layer shot map,thereby measuring array coordinates (Mxn,Myn) in a stage coordinatesystem (i.e. a coordinate system determined by values measured with thelaser interferometers 7B and 9B of the middle stepper 1B) of each of thewafer marks 221X and 221Y attached to 10 (for example) shot areas(sample shots) SEa, SEb, SEc, . . . , SEj selected from among the shotareas on the wafer 20, as shown in FIG. 19(a). Then, values of the sixEGA parameters (scaling parameters Rx and Ry, rotation θ,perpendicularity W, and offsets Ox and Oy) in Eq. (1) are determined soas to minimize the residual error component (expressed by Eq. (2)) ofthe alignment error, that is, the deviation of the measured values(Mxn,Myn) of the wafer marks 221X and 221Y of each sample shot from thearray coordinate values, which are calculated from the design arraycoordinates (Dxn,Dyn) of the wafer marks 221X and 221Y.

Next, the controller 10B sequentially substitutes the six EGA parametersand the design array coordinate values (Dxm,Dym) of the shot areas SEm(m=1 to M) into the right-hand side of Eq. (1), thereby obtaining arraycoordinate values in the stage coordinate system of each shot area SEmof the critical layer on the wafer 20. At this time, because theexposure field 54B of the middle stepper 1B is twice as large as theexposure field 54A of the fine stepper 1A in both the directions X andY, the controller 10B divides the shot areas SEm (m−1 to M), shown inFIG. 19(a), into a plurality of blocks each comprising an array of twoshot areas in the direction X and two shot areas in the direction Y, andobtains array coordinates in the stage coordinate system of the centerof each block from the computational array coordinates of the four shotareas in the block. Thereafter, the controller 10B sequentially alignsthe array coordinates of the center of each block on the wafer 20 withthe center of the exposure field 54B, and transfers an image of thevernier mark original drawing patterns formed on the reticle RB ontoeach block. Thereafter, development is carried out, thereby allowingmiddle layer vernier marks to appear over the critical layer verniermarks on the wafer 20. It should be noted that the following measurementmay be effected with the transferred marks left in the form of latentimages, as has already been described above.

Next, a third step will be explained.

In the third step, an amount of positional displacement between thecritical and middle layer vernier marks is measured. For this purpose,the wafer 20 having been subjected to development in the second step isplaced, for example, on the wafer stage 5B of the middle stepper 1B,shown in FIG. 18, and an amount of positional displacement between thecritical and middle layer vernier marks is measured by the alignmentsystem 11B. However, the measurement of a positional displacementbetween the critical and middle layer vernier marks may be carried outby using another measuring device of high accuracy.

FIG. 20(a) shows the wafer 20 having overlaid vernier marks formed bythe exposure process in the second step. In FIG. 20(a), the wafer 20 hasmiddle layer shot areas SF1, SF2, . . . , SFN (N is an integer of 4 ormore) arranged at a pitch 2 d along the X-axis and at a pitch 2 c alongthe Y-axis. Each shot area SFn (n=1 to N) contains four critical layershot areas SEm. It should be noted that, if there is a magnificationerror in each shot area SFn of the middle layer, the widths of each shotarea SFn in the directions X and Y are slightly deviated from 2 d and 2c, respectively. Further, the center 261 of each shot area SFn issubstantially coincident with the center of the associated four criticallayer shot areas. Each shot area SFn has a total of 36 (=4×9) verniermarks corresponding to the nine vernier marks 222A to 222E and 223A to223D (see FIG. 19(b)) in each critical layer shot area SEm.

Assuming that each middle layer shot area SFn is M₁/N₁ times and M₂/N₂times as large as the critical layer shot area SEm in the directions Xand Y, respectively, in this embodiment M₁/N₁=2/1 and M₂/N₂=2/1.Accordingly, a reference measurement area in this embodiment is an areadetermined by multiplying the middle layer shot area SFn by one in eachof the directions X and Y, that is, the shot area SFn itself. Therefore,four shot areas (reference measurement areas) SFa to SFd which aresubstantially uniformly distributed over the wafer 20, as shown by thehatching in the figure, are defined as objects to be measured.

FIG. 20(b) shows the shot area SFa among the four reference measurementareas. In FIG. 20(b), the shot area SFa has nine middle layer verniermarks 224A to 224E and 225A to 225D formed to surround the respectivevernier marks which belong to the second-quadrant shot area SEp of thefour critical layer shot areas underlying the middle layer shot areaSFa. The shot area SFa further has nine vernier marks (not shown)similarly formed to surround the respective vernier marks which belongto each of the other shot areas SE(p+1), SEq and SE(q+1) underlying theshot area SFa. However, FIG. 20(b) shows the middle layer vernier-mark226C corresponding to the vernier mark 222C formed in the intermediateportion at the right end of the first-quadrant shot area SE(p+1) amongthose middle layer vernier marks.

Next, in this embodiment, an amount of positional displacement betweenthe critical layer vernier mark 222C and the middle layer vernier mark226C is measured at each of measuring points 232A to 232D lying at themutually identical positions in the shot areas (reference measurementareas) SFa to SFd, which are objects to be measured, on the wafer 20.For example, the measuring points 232A to 232D each lies in theintermediate portion at the right end of the first quadrant [i.e. anarea corresponding to the shot area SE(p+1) in FIG. 20(b)] in the shotareas SFa to SFd. Accordingly, at the measuring point 232A, positionaldisplacements (Δxa,Δya) in the directions X and Y of the vernier mark226C relative to the vernier mark 222C is measured. At the othermeasuring points 232B to 232D, positional displacements (Δxb,Δyb) to(Δxd,Δyd) are similarly measured.

Thereafter, if there is a difference (Δxb−Δxd) between the positionaldisplacements in the direction X measured at the two measuring points232D and 232B in FIG. 20(a), for example, the difference (Δxb−Δxd) isdivided by the distance in the direction X between the two measuringpoints 232D and 232B, thereby obtaining a correction value (error) ΔRxfor the scaling parameter Rx in the direction X among the EGAparameters. If there is a difference (Δyb−Δyd) between the positionaldisplacements in the direction Y measured at the measuring points 232Dand 232B, the difference (Δyb−Δyd) is divided by the distance in thedirection X between the two measuring points, thereby obtaining acorrection value Δθ for the rotation θ among the EGA parameters.Further, mean values of the positional displacements in the directions Xand Y measured at the four measuring points are defined as correctionvalues ΔOx and ΔOy for the offsets Ox and Oy among the EGA parameters.Similarly, correction values ΔRy and ΔW for the other EGA parameters,that is, the scaling parameter Ry and the perpendicularity W, are alsoobtained. These correction values are stored in a storage unit in thecontroller 10B of the middle stepper 1B. It should be noted that, ifpositional displacements between the corresponding vernier marks aremeasured with another measuring device, and correction values areobtained by using another computer or the like, the operator inputs thecorrection values to the controller 10B through an input device. Thus,the third step is terminated.

Thereafter, in a case where exposure is carried out by the mix-and-matchmethod using the fine stepper 1A and the middle stepper 1B, shown inFIG. 18, a critical layer pattern is formed on the wafer 20 by using thefine stepper 1A, and before a middle layer pattern is formed by usingthe middle stepper 1B, coordinate values of predetermined sample shotsin the stage coordinate system are measured, and values of the six EGAparameters in Eq. (1) are determined on the basis of the measuredcoordinate values. Thereafter, the controller 10B adds the EGA parametercorrection values (ΔRx, ΔRy, Δθ, ΔW, ΔOx, and ΔOy), stored in theabove-described third step, to the determined EGA parameters (Rx, Ry, θ,W, Ox, and Oy) to obtain corrected EGA parameters. Then, the controller10B calculates coordinate positions of the shot areas of the criticallayer by using the corrected EGA parameters, calculates exposurepositions of the shot areas of the middle layer on the basis of thecoordinate positions of the critical layer shot areas, and sequentiallytransfers the reticle pattern for the middle layer onto the middle layershot areas on the basis of the exposure positions.

In this embodiment, the measuring points used in the above-describedthird step are at the mutually identical positions in the shot areas(reference measurement areas) SFa to SFd, as shown in FIG. 20(a).Accordingly, even when the middle layer shot areas SFn have amagnification error or a rotation error, the same offset value issuperimposed at each measuring point, and the magnification or rotationerror affects only the offsets Ox and Oy among the EGA parameters. Thus,the overlay accuracy between the critical and middle layers improvesbecause the values of other influential linear parameters (Rx, Ry, θ,and W) are accurate.

In the above-described embodiment, a magnification error or rotationerror in the middle layer shot areas affects the offsets Ox and Oy amongthe EGA parameters. Therefore, the effect of the magnification orrotation error is eliminated by averaging process. The method ofeliminating the magnification or rotation error will be explained belowwith reference to FIGS. 21(a) to 23(b), in which portions correspondingto those in FIGS. 20(a) and 20(b) are denoted by the same referencecharacters.

FIG. 21(a) shows a first method of arranging measuring points foreliminating an offset error. In FIG. 21(a), four middle layer shot areasSFa to SFd are selected as reference measurement areas on the wafer 20in the same way as in FIG. 20(a). Then, an amount of positionaldisplacement between two corresponding vernier marks is measured at eachof four measuring points in the shot area SFa, that is, a measuringpoint 233A at the bottom left in the first quadrant, a measuring point235A at the bottom right in the second quadrant, a measuring point 234Aat the top right in the third quadrant, and a measuring point 236A atthe top left in the fourth quadrant. Mean values of the positionaldisplacements in the directions X and Y measured at the four measuringpoints are assumed to be (Δxa′,Δya′).

More specifically, FIG. 21(b) is an enlarged view of the shot area SFa.As shown in FIG. 21(b), an amount of positional displacement between thevernier mark 223D in the shot area SE(p+1) and the middle layer verniermark 227D is measured at the measuring point 233A, and an amount ofpositional displacement between the vernier mark 223C in the shot areaSEp and the middle layer vernier mark 225C is measured at the measuringpoint 235A. Similarly, an amount of positional displacement between thevernier mark 223B (or 223A) and the vernier mark 229B (or 231A) ismeasured at the measuring point 234A (or 236A).

Referring to FIG. 21(a), positional displacement is similarly measuredin each of the other shot areas SFb to SFd. That is, in each shot area,an amount of positional displacement between the two correspondingvernier marks is similarly measured at each of the four measuring pointslying at respective positions mutually identical with the measuringpoints 233A to 236A in the shot area SFa, and mean values of themeasured amounts of positional displacement are determined to be(Δxb′,Δyb′) to (Δxd′,Δyd′). Thereafter, EGA parameter correction valuesare obtained from the amounts of positional displacement measured in thefour shot areas SFa to SFd. In this case, even if the shot area SFa, forexample, has a magnification error or rotation error (shot rotationerror), the effect of such an error appears symmetrically at the fourmeasuring points 233A to 236A; therefore, the effect of themagnification or rotation error can be eliminated by averaging theamounts of positional displacement at the four measuring points.Accordingly, even if there is a magnification or rotation error, noerror will be introduced into the offsets Ox and Oy in the EGAparameters.

Further, the exposure method according to this embodiment provides thefollowing advantageous effects: Since the measuring points 233A to 236Alie in the center of the shot area SFa, the distortion introduced by theprojection optical system 3B of the middle stepper 1B is small at themeasuring points 233A to 236A, and thus the distortion of the middlelayer shot areas produces a minimal effect on the measurement result.Further, in the shot area SFa, for example, measurement is carried outin each of four different corners of the four critical layer shot areas.Therefore, the effects of the magnification or rotation errors in thecritical layer shot areas are canceled by the averaging process.Similarly, the effect of the distortion of the critical layer shot areasis reduced by the averaging process.

In this embodiment, it is only necessary to enable measuring points tobe symmetrically disposed in each middle layer shot area used as areference measurement area. Therefore, in FIG. 21(a), only two measuringpoints shown by the black circles may be selected from each of the shotareas SFa to SFd, for example, (i.e. the measuring points 233A and 234Ain the shot area SFa). Alternatively, only two measuring points shown bythe white circles may be selected from each of the shot areas SFa to SFd(i.e. the measuring points 235A and 236A in the shot area SFa).

Although in the above-described embodiment the measuring points areconcentrated on the center of each of the middle layer shot areas usedas reference measurement areas, the arrangement of measuring points isnot necessarily limited to it. As shown in FIG. 22(a), measuring points237A to 240A may be set in the respective centers of the four criticallayer shot areas in the shot area SFa, for example. It is also possibleto select two measuring points 237A and 238A, shown by the blackcircles, or two measuring points 239A and 240A, shown by the whitecircles, from among the four measuring points. In this case, thedistortion of the critical layer shot areas is minimized, and the effectof the distortion of the middle layer shot areas is reduced by theaveraging process.

However, in a case where the distortion of the critical and middle layershot areas has previously been known to be small, as shown for examplein FIG. 22(b), four measuring points 241A to 244A in the four corners ofthe shot area SFa may be selected. Alternatively, only two measuringpoints 241A and 242A, shown by the black circles, or only two measuringpoints 243A and 244A, shown by the white circles, may be selected.

To sum up, an efficient measuring point layout which enables theaveraging effect to be obtained and which makes it possible to minimizethe number of measuring points and to shorten the time required formeasurement is such as that shown, for example, in FIG. 23(a) or 23(b).In the layout shown in FIG. 23(a), measuring points 233A to 233D and234A to 234D are selected in four shot areas (reference measurementareas) SFa to SFd on the wafer 20. More specifically, the measuringpoints 233A to 233D are each on the right side of the center of theassociated shot area, toward the top as viewed in the figure, and themeasuring points 234A to 234D are each on the left side of the center ofthe associated shot area, toward the bottom as viewed in the figure. Inthe layout shown in FIG. 23(b), measuring points are selected asfollows: In a pair of mutually opposing shot areas SFa and SFc among thefour shot areas, measuring points 235A and 235C are selected which areeach on the left side of the center of the associated shot area, towardthe top, and measuring points 236A and 236C are selected which are eachon the right side of the center of the associated shot area, toward thebottom; in the other pair of mutually opposing shot areas SFb and SFd,measuring points 233B and 233D are selected which are each on the rightside of the center of the associated shot area, toward the top, andmeasuring points 234B and 234D are selected which are each on the leftside of the center of the associated shot area, toward the bottom.

It is also possible to select from each reference measurement area onemeasuring point which is at a symmetric position with respect to thecenter of the area, as shown in FIGS. 24(a) and 24(b). That is, FIG.24(a) shows middle layer shot areas transferred over a critical layer onthe wafer 20. From among the shot areas, eight shot areas SFa to SFh,which are substantially uniformly distributed, are selected as referencemeasurement areas. The shot areas SFa to SFh each contains four criticallayer shot areas.

Then, from the two shot areas SFc and SFg, measuring points 233C and233G are respectively selected which are each on the right side of thecenter of associated shot area, toward the top, and from the two shotareas SFa and SFe, measuring points 235A and 235E are respectivelyselected which are each on the left side of the center of the associatedshot area, toward the top. From the two shot areas SFd and SFh,measuring points 234C and 234H are respectively selected which are eachon the left side of the center of the associated shot area, toward thebottom, arid from the other two shot areas SFb and SFf, measuring points236B and 236F are respectively selected which are each on the right sideof the center of the associated shot area, toward the bottom. Then, anamount of positional displacement between the critical and middle layervernier marks is measured at each of the selected measuring points. Inthis example, thereafter, the amounts of positional displacementmeasured at two measuring points which are at symmetric positions in apair of middle layer shot areas (e.g. the measuring points 235A and236B) are averaged, thereby reducing the effect of the magnification orrotation error of the middle layer. The effects of the middle layerdistortion, the reticle writing error, etc. are also reduced by theaveraging process.

Let us consider the measuring method in this example in terms of onecritical layer shot area SE, as shown in FIG. 24(b). In this example,measurement is carried out twice at each of measuring points 245A to245D in the four corners of the shot area SE. Therefore, assuming thatthe magnification or rotation error of the critical layer shot areas issubstantially uniform over the wafer, it is possible to reduce theeffects of magnification error, rotation error, and distortion of thecritical layer shot areas, reticle writing error, etc. by averaging theamounts of positional displacement measured, for example, at a pair ofmutually opposing measuring points (e.g. the measuring points 245A and245C) among the measuring points 245A to 245D in the four corners of theshot area SE.

As has been described above, the third embodiment shows a measuringpoint layout which is applicable in a case where the size of each middlelayer shot area is twice as large as each critical layer shot area ineach of the directions X and Y, and where one chip pattern, for example,is formed in each critical layer shot area. In actuality, however, twoor more chip patterns may be contained in each critical layer shot area;there are various size ratios of the middle layer shot areas to thecritical layer shot areas. Further, projection exposure apparatusesusable in the third embodiment are not necessarily limited to one-shotexposure type projection exposure apparatuses such as steppers; it isalso possible to use scanning exposure type projection exposureapparatuses, e.g. step-and-scan type projection exposure apparatuses inwhich a pattern on a reticle is sequentially transferred onto each shotarea on a wafer by synchronously scanning the reticle and the wafer withrespect to a projection optical system. Various other modifications ofthe third embodiment will be explained below with reference to FIGS.25(a) to 27(b).

In the modification shown in FIGS. 25(a) to 25(c), each critical layershot area SE has, as shown in FIG. 25(a), two identical chip patterns246A and 246B arranged in the direction Y. As shown in FIG. 25(b), eachmiddle layer shot area SF has identical chip patterns arranged in twocolumns in the direction X and four rows in the direction Y. In thiscase, assuming that each chip pattern is a rectangular pattern having awidth b in the direction X and a width a in the direction Y, the widthin the direction X of the critical layer shot area SE is b, and thewidth in the direction Y of the shot area SE is 2 a. The width in thedirection X of the middle layer shot area SF is 2 b, and the width inthe direction Y of the shot area SF is 4 a. Accordingly, the shot areaSF is 2/1 times as large as the shot area SE in each of the directions Xand Y. Therefore, as shown in FIG. 25(c), a reference measurement areaSG, which has a size regarded as being the least common multiple of thesizes of the shot areas SE and SF, has a width 2 b in the direction Xand a width 4 a in the direction Y. That is, the reference measurementarea SG has the same size as that of the middle layer shot area SF.Accordingly, when a measuring point 247, for example, is selected in acertain reference measurement area SG, in the other referencemeasurement areas also measuring points which are at the identicalpositions with the measuring point 247 are selected. By doing so, EGAparameter correction values can be accurately obtained.

However, in order to reduce the effect of the magnification error,rotation error, etc. of the middle layer shot areas, it is desirable toselect, for example, measuring points which are in symmetric relation tothe measuring point 247 with respect to the center position in thereference measurement areas in the same way as in the above-describedthird embodiment. The same is true of the following modifications.

In the modification shown in FIGS. 26(a) to 26(c), a critical layer shotarea SE has, as shown in FIG. 26(a), two identical chip patternsarranged in the direction Y. As shown in FIG. 26(b), a middle layer shotarea SH has three identical chip patterns arranged in the direction Y.Further, the middle layer projection exposure apparatus is of thescanning exposure type. Thus, the shot area SH is exposed by scanningthe wafer with respect to a slit-shaped exposure area 248.

At this time, assuming that the critical layer shot area SS has a widthb in the direction X and a width 2 a in the direction Y, the width inthe direction X of the middle layer shot area SH is b, and the width inthe direction Y of the shot area SH is 3 a. Accordingly, the shot areaSH is 1/1 time as large as the shot area SE in the direction X, and theformer is 3/2 times as large as the latter in the direction Y.Therefore, as shown in FIG. 26(c), a reference measurement area SI,which has a size regarded as being the least common multiple of thesizes of the shot areas SE and SH, has a width b in the direction X anda width 6 a in the direction Y. In this modification also, when ameasuring point 249, for example, is selected in a certain referencemeasurement area SI, in the other reference measurement areas alsomeasuring points which are at the identical positions with the measuringpoint 249 are selected. By doing so, EGA parameter correction values canbe accurately obtained.

In this regard, FIG. 27(a) shows an enlarged view of one example of thereference measurement area SI, shown in FIG. 26(c). In FIG. 27(a), apair of adjacent shot areas SH1 and SH2 exposed by the scanning exposuremethod contain three critical layer shot areas SE1, SE2 and SE3. FIG.27(b) shows an expansion and contraction quantity ΔY in the longitudinaldirection (direction Y) in the shot areas SH1 and SH2, shown in FIG.27(a), due to a magnification error. The expansion and contractionquantity ΔY in the direction Y changes at a period which is equal to thelength of each of the shot areas SH1 and SH2. Accordingly, if thecenters of the critical layer shot areas SE1 to SE3 are defined asmeasuring points 250A to 250C, for example, the expansion andcontraction quantities of the middle layer shot areas measured at themeasuring points 250A to 250C show different values as shown by thepositions 251A to 251C in FIG. 27(b). Accordingly, if a given measuringpoint 249 is selected in a certain reference measurement area SI in FIG.26(c), the measurement result is affected by the magnification error ofthe middle layer shot areas unless measuring points are selected at theidentical positions with the measuring point 249 in the other referencemeasurement areas.

In the modification shown in FIGS. 28(a) to 28(c), a critical layer shotarea SF has, as shown in FIG. 28(a), identical chip patterns arranged inthree rows in the direction Y and two columns in the direction X. Asshown in FIG. 28(b), a middle layer shot area SH exposed by the scanningexposure method has three identical chip patterns arranged in thedirection Y. In this case, assuming that the width in the direction X ofthe critical layer shot area SF is 2 b, and the width in the direction Yof the shot area SF is 3 a, the width in the direction X of the middlelayer shot area SH is b, and the width in the direction Y of the shotarea SH is 3 a. Accordingly, as shown in FIG. 28(c), a referencemeasurement area SJ, which has a size regarded as being the least commonmultiple of the sizes of the shot areas SF and SH, has a width 2 b inthe direction X and a width 3 a in the direction Y. That is, thereference measurement area SJ has the same size as that of the criticallayer shot area SF. In this modification also, when a measuring point252, for example, is selected in a certain reference measurement areaSJ, in the other reference measurement areas also measuring points whichare at the identical positions with the measuring point 252 areselected. By doing so, EGA parameter correction values can be accuratelyobtained.

Although in the above-described third embodiment and modificationsthereof a combination of two steppers or a combination of a stepper anda step-and-scan type projection exposure apparatus is used, it should benoted that a combination of usable projection exposure apparatuses isnot necessarily limited to the above. For example, it is also possibleto use two different step-and-scan type projection exposure apparatusesas an exposure apparatus having a small exposure field and an exposureapparatus having a large exposure field.

According to the exposure method in the third embodiment, an area whichis so large as to contain an integer number of first and second exposurefields in each of two directions (i.e. an area having a size regarded asbeing the least common multiple of the sizes of the first and secondexposure fields) is defined as a reference measurement area, and anamount of positional displacement between two corresponding overlayaccuracy measuring marks (i.e. vernier marks) is measured at each ofmeasuring points lying at the mutually identical positions in apredetermined number of reference measurement areas. Therefore, there isno possibility that the effect of a magnification or rotation error, forexample, of the second mask pattern will appear as a linear expansionand contraction error or rotation error in alignment errors which mayarise during the exposure of the second mask pattern. Accordingly, it ispossible to increase the overlay accuracy between a critical layerpattern and a middle layer pattern in a case where exposure is carriedout by the mix-and-match method with respect to a substrate where acritical layer End a middle layer are mixedly present.

In a case where the second exposure apparatus calculates an exposureposition by using coordinate transformation parameters and obtainscorrection values for the parameters from results of measurement carriedout for each reference measurement area, the overlay accuracy can beincreased because a magnification or rotation error of the second maskpattern has no effect on parameters indicating linear expansion andcontraction, rotation and perpendicularity among the coordinatetransformation parameters.

Regarding offset parameters, the effect of a magnification or rotationerror of the second mask pattern can be reduced, for example, by usingmean values of results of measurement carried out at measuring pointsdisposed symmetrically with respect to the center point in the referencemeasurement areas.

Next, one example of a fourth embodiment of the exposure methodaccording to the present invention will be described with reference toFIGS. 29 to 32. Two exposure apparatuses used in this example are aone-shot exposure type projection exposure apparatus (stepper) with ademagnification ratio of 5:1 and a step-and-scan type projectionexposure apparatus with a demagnification ratio of 4:1. In this example,two chip patterns are formed in each shot area exposed by the formerprojection exposure apparatus (i.e. a two-chip reticle is used), andthree chip patterns are formed in each shot area scan-exposed by thelatter projection exposure apparatus (i.e. a three-chip reticle isused). It should be noted that constituent elements in this examplewhich are similar to those in the first to third embodiments are denotedby the same reference characters and will be briefly explained below.

FIG. 29 shows an exposure system used in this embodiment. In FIG. 29, aone-shot exposure type projection exposure apparatus (hereinafterreferred to as “fine stepper”) 1A, and a step-and-scan type projectionexposure apparatus (hereinafter referred to as “scanning exposureapparatus”) 1B are installed. In this embodiment, the fine stepper 1A isa high-resolution exposure apparatus, while the scanning exposureapparatus 1B is a low-resolution exposure apparatus. The fine stepper 1Ais used to carry out exposure for a critical layer on a wafer, and thescanning exposure apparatus 1B is used to carry out exposure for amiddle layer on the wafer.

First, in the fine stepper 1A, a pattern area 62A on a reticle RA isilluminated by exposure light from an illumination optical system (notshown), and an image of a pattern formed in the pattern area 62A isformed on a rectangular exposure field 64A on a wafer 20 as a projectedimage reduced to ⅕ by a projection optical system 3A. A Z1-axis is takenin a direction parallel to an optical axis of the projection opticalsystem 3A, and two axes of an orthogonal coordinate system set in aplane perpendicular to the Z1-axis are defined as an X1-axis and aY1-axis, respectively. The pattern area 62A on the reticle RA is dividedinto partial pattern areas 312A and 312B of the same size in thedirection Y1. The partial pattern areas 312A and 312B each has originaldrawing patterns of alignment marks and overlay accuracy measuring marks(vernier marks) written according to the same layout.

A wafer stage 5A comprises a Z-stage, an XY-stage, etc. The coordinatein the direction X1 of the wafer stage 5A is measured by a combinationof a moving mirror 6A and a laser interferometer 7A. The coordinate inthe direction Y1 of the wafer stage 5A is measured by a combination of amoving mirror 8A and a laser interferometer 9A. The coordinates measuredby the laser interferometers 7A and 9A are supplied to a controller 10Awhich controls operations of the whole apparatus. The controller 10Adrives the wafer stage 5A to step, thereby positioning the wafer 20. Thestepping drive of the wafer 20 is effected according to a shot map for acritical layer. The shot map is generated by a map generating unit whichcomprises a computer in the controller 10A.

An off-axis imaging type (FIA type) alignment system 11A images analignment mark (wafer mark) on the wafer 20 to detect X1 and Y1coordinates of the mark. The detected coordinates are supplied to thecontroller 10A.

Next, in the scanning exposure apparatus 1B in this example, a part of apattern area 62B on a reticle RB is illuminated by exposure light froman illumination optical system (not shown), and an image of a part ofthe reticle pattern is formed in a slit-shaped exposure area 314 on awafer 20, which is held on a wafer stage 5B, as a projected imagereduced to ¼ by a projection optical system 3B. The reticle RB isscanned in the direction −Y2 (or +Y2), and the wafer 20 is scanned inthe direction +Y2 (or −Y2) in synchronism with the scanning of thereticle RB, thereby sequentially projecting an image of the patternformed in the pattern area 62B of the reticle RB onto the exposure field64B on the wafer 20.

The pattern area 62B of the reticle RB is divided into three partialpattern areas 313A to 313C of the same size in the direction Y2, whichis the scanning direction. The size of the exposure field 64B is suchthat its dimension in the scanning direction is 3/2 times the dimensionof the exposure field 64A of the fine stepper 1A, and the exposure field64B is equal in size (1:1) to the exposure field 64A in the non-scanningdirection. The partial pattern areas 313A to 313C also each has originaldrawing patterns of vernier marks formed according to the same layout,

The position of a reticle stage (not shown) for scanning the reticle RBof the scanning exposure apparatus and the X2 and Y2 coordinates of thewafer stage 5B are supplied to a controller 10B. The controller 10Bcontrols synchronous drive of the reticle stage (not shown) and thewafer stage 5B. The scanning exposure operation of the wafer stage 5B iseffected according to a shot map for a middle layer set on an exposuresurface of the wafer 20, which is to be exposed. The shot map isgenerated by a map generating unit which comprises a computer in thecontroller 10B.

In this case, the map generating unit in the controller 10A and the mapgenerating unit in the controller 10B have the function of supplying.shot map information prepared thereby to each other.

The scanning exposure apparatus 1B also has an off-axis imaging type(FIA type) alignment system 11B provided at a side surface of theprojection optical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark or vernier mark on the wafer 20.

Next, one example of an operation of correcting in-shot parameters (i.e.shot magnifications rx and ry, shot rotation θ, and shotperpendicularity w) when exposure of the pattern for the middle layer isto be effected by the scanning exposure apparatus 1B after exposure ofthe pattern for the critical layer has been carried out by the finestepper 1A in this example will be explained for each of the first tothird processing steps.

First, the first step will be explained.

In the first step, an unexposed wafer 20 coated with a photoresist isplaced on the wafer stage 5A of the fine stepper 1A, shown in FIG. 29,and a reduced image of the pattern on the reticle RA is sequentiallytransferred by the step-and-repeat method onto a multiplicity of shotareas arrayed on the wafer 20 in units of the exposure field 64A. Thereticle RA has original drawing patterns of two sets of vernier marksformed according to a predetermined layout in addition to two pairs ofalignment marks. Thereafter, the wafer 20 is subjected to development,thereby allowing the two pairs of alignment marks to appear as wafermarks comprising recess-and-projection patterns, and also allowing thetwo sets of vernier mark original drawing patterns to appear as verniermarks comprising recess-and-projection patterns. The patterns obtainedafter the development can be regarded as critical layer patterns on thewafer 20. However, it is also possible to carry out the followingalignment and measurement of an amount of positional displacementbetween two corresponding vernier marks with these marks left in theform of latent images without effecting development.

Next, the second step will be explained.

A photoresist is coated over the wafer 20 having the wafer and verniermarks formed in the first step, and the photoresist-coated wafer 20 isplaced on the wafer stage 5B of the scanning exposure apparatus 1B,shown in FIG. 29. At this time, information concerning the criticallayer shot map used in the first step has been supplied from thecontroller 10A of the fine stepper 1A to the controller 10B of thescanning exposure apparatus 1B. Thus, the controller 10B can obtaindesign array coordinates of the critical layer wafer and vernier markson the wafer 20.

FIG. 30(a) shows the wafer 20 placed on the wafer stage 5B. In FIG.30(a), the X2- and Y2-axes of the scanning exposure apparatus 1B areshown as being X- and Y-axes, respectively. In this case, the wafer 20has been roughly aligned by a pre-alignment mechanism (not shown), andthe surface of the wafer 20 has been divided into Q (Q=32 in FIG. 30(a))critical layer shot areas SM1, SM2, . . . , SMQ in two directions whichare approximately parallel to the directions X and Y, respectively. Inactual practice, a scribe line area of a predetermined width liesbetween shot areas SMq (q=1 to Q); however, illustration of the scribeline area is omitted in FIG. 30(a). The width (pitch) in the direction Xof each shot area SMq, including the scribe line area, is b, and thewidth (pitch) in the direction Y is 2 a. In this embodiment, each shotarea SMq is approximately square (2 a≈b). Further, each shot area SMq isdivided into two partial shot areas of the same shape in the directionY, that is, first and second partial areas 315A and 315B, in whichcircuit patterns identical with each other are to be formed.

FIG. 30(b) shows a shot area SMq as a typical example of the criticallayer shot areas. In FIG. 30(b), the first partial shot area in the shotarea SMq is provided with a pair of wafer marks 321XA and 321YA for theX- and Y-axes, and also one set of four vernier marks 331A, 331C, 331Dand 331E which are distributed in a cross shape. Similarly, the secondpartial shot area in the shot area SMq is provided with a pair of wafermarks 321XB and 321YB and four vernier marks 332A, 332C, 332D and 332Ein symmetric relation to the marks in the first partial shot area. Inthis case, the original drawing patterns of marks distributed as shownin FIG. 30(b) have been formed in the pattern area 62A of the reticle RAin the fine stepper 1A, shown in FIG. 29.

It should be noted that the wafer marks 321XA to 321YA, etc. used inthis example are one-dimensional line-and-space patterns which aredetected by an imaging detection method with the alignment system 11B,shown in FIG. 29. The vernier marks 331A to 331E, etc. aretwo-dimensional box-in-box marks which are detected by an imagingdetection method with the alignment system 11B. However, it is possibleto use other kinds of mark as vernier marks, for example, marks eachformed by a combination of two one-dimensional line-and-space patternswhich are crossed at right angles. It is also possible to use the wafermarks 321XA, 321YA, etc. themselves as vernier marks. Conversely,vernier marks may be used as wafer marks. In this embodiment, a part ofthe vernier marks 331A to 331E and 332A to 332E are used as multipointwafer marks (alignment marks) in the shot area SMq as one example.Further, marks which are detected by the laser step alignment (LEA)method, for example, may also be used as vernier marks. The distributionof wafer and vernier marks is not necessarily limited to that shown inFIG. 30(b).

Next, the controller 10B of the scanning exposure apparatus 1B, shown inFIG. 29, effects alignment by the EGA method. Accordingly, thecontroller 10B drives the wafer stage 5B to move the field of view ofthe alignment system 11B sequentially according to the critical layershot map, thereby measuring array coordinates in a stage coordinatesystem (i.e. a coordinate system determined by values measured with thelaser interferometers 7B and 9B of the scanning exposure apparatus 1B)of each of the wafer marks 321XA and 321YA attached to nine (forexample) shot areas (sample shots) S310, S320, . . . , S390 selectedfrom among the shot areas on the wafer 20, as shown in FIG. 30(a). Then,values of six EGA parameters (scaling parameters Rx and Ry, waferrotation θ, shot perpendicularity W, and offsets Ox and Oy) on the wafer20 are determined so as to minimize the residual error component, whichis the sum of the squares of deviations of the measured values of thewafer marks 321XA and 321YA of each sample shot from array coordinatevalues calculated from the design array coordinates of the wafer marks321XA and 321YA.

Next, the controller 10B determines array coordinate values of each shotarea SMq (q=1 to Q) in the stage coordinate system from the six EGAparameters and the design array coordinate values of the critical layershot area SMq. In this case, the exposure field 64B of the scanningexposure apparatus 1B is equal in size (1:1) to the exposure field 64Aof the fine stepper 1A in the direction X but 3/2 times as large as theexposure field 64A in the direction Y. Therefore, the controller 10Bdivides perfect partial shot areas (with no missing part) in the shotareas SMq (q=1 to Q), shown in FIG. 30(a), into a plurality of blockseach comprising one partial shot area in the direction X and threepartial shot areas in the direction Y, each block containing at leastone shot area SMq. Then, the controller 10B obtains array coordinates ofthe center of each block in the stage coordinate system from thecomputational array coordinates of the shot area SMq contained in theblock. Thus, an array (shot map) of middle layer shot areas isdetermined.

For example, in the shot map for the middle layer to be exposed by thescanning exposure apparatus 1B, as shown in FIG. 31(a), R (R=20 in FIG.31(a)) shot areas SN1, SN2, . . . , SNR are arranged in the directions Xand Y over the critical layer on the wafer 20. The width (pitch) in thedirection X of each shot area SNr (r=1 to R), including the scribe linearea, is b, and the width (pitch) in the direction Y is 3 a.Accordingly, assuming that the size of the middle layer shot area SNr inthe direction X is M₁/N₁ times as large as that of the critical layershot area SMq, and the size of the shot area SNr in the direction Y isM₂/N₂ times as large as that of the shot area SMq, the size ratios inthis embodiment are M₁/N₁=1/1 and M₂/N₂=3/2. Further, each shot area SNris divided into three partial shot areas 316A to 316C of the same sizein the direction Y (i.e. the scanning direction). The three partial shotareas 316A to 316C are to be formed with identical patterns.

In this embodiment, it is assumed that six EGA parameters (Rx, Ry, θ, W,Ox, and Oy) have no error, but four in-shot parameters (shotmagnifications rx and ry, shot rotation θ, and shot perpendicularity w)have errors. Therefore, overlay exposure is carried out in order toobtain errors (correction values) of these in-shot parameters.

That is, the scanning exposure apparatus 1B sequentially transfers animage of the vernier mark original drawing patterns formed on thereticle RB onto each of the middle layer shot areas SNr, shown in FIG.31(a), by the scanning exposure method. Prior to the exposure process,the projection magnification and scanning speed of the projectionoptical system 3B have been adjusted according to the calculated shotmagnifications rx and ry, and the reticle RB has been rotated accordingto the shot rotation θ. Further, the scanning direction has beenadjusted according to the shot perpendicularity w. Thus, the middlelayer chip pattern has previously been aligned with respect to thecritical layer chip pattern. After the exposure process, development iscarried out, thereby allowing middle layer vernier marks to appear overthe critical layer vernier marks on the wafer 20. It should be notedthat the following measurement may be carried out with the transferredmarks left in the form of latent images, as has already been describedabove.

Next, the third step will be explained.

In the third step, measurement is carried out to determine amounts ofpositional displacement between the corresponding vernier marks in thecritical layer shot areas SMq, shown in FIG. 30(a), and the middle layershot areas SNr, shown in FIG. 31(a). For this purpose, the wafer 20,shown in FIG. 31(a), which has been subjected to the development in thesecond step, is placed, for example, on the wafer stage 5B of thescanning exposure apparatus 1B, shown in FIG. 29, and amounts ofpositional displacement between the corresponding vernier marks on thetwo layers are measured by the alignment system 11B. However, themeasurement of positional displacement between the corresponding verniermarks may be carried out by using another measuring device of highaccuracy.

In this case, it is assumed that, as shown in FIGS. 30(a) and 31(a), the+Y direction end of the array of the critical layer shot areas SMq iscoincident with the +Y direction end of the array of the middle layershot areas SNr. The shot areas SN1 to SNR of the middle layer M are eachprovided with 12 (=4×3) vernier marks respectively corresponding to thevernier marks in the critical layer shot areas SMq (each having 8vernier marks).

FIG. 31(b) shows a middle layer shot area SNr. In FIG. 31(b), the firstpartial shot area in the shot area SNr has four vernier marks 333A to333E formed so as to surround the critical layer vernier marks 331A to331E (see FIG. 30(b)), respectively. Similarly, the second and thirdpartial shot areas in the shot area SNr have four vernier marks 334A to334E and four vernier marks 335A to 335E, respectively, formed so as tosurround the corresponding critical layer vernier marks. In thisexample, it is assumed, for example, that the middle layer vernier mark335E is displaced by Δx and Δy in the directions X and Y relative to thevernier mark 331E in a predetermined critical layer shot area due toerrors of the four in-shot parameters (rx, ry, θ, and w). Accordingly,errors (correction values) of the in-shot parameters are obtained bydetecting amounts of positional displacement between the correspondingvernier marks on the two layers at predetermined measuring points.

A method of setting measuring points in two shot areas SNr and SN(r+1)which are contiguous with each other in the direction Y, as shown forexample by the hatching in FIG. 31(a), will be explained below withreference to FIG. 32.

Referring to FIG. 32, areas in each of which one of the middle layershot areas SNr and SN(r+1) and one of the critical layer shot areas SMq,SM(q+1) and SM(q+2) are perfectly overlaid on one another such thatneither of the overlaid shot areas extends over a plurality of middle orcritical layer shot areas, that is, two hatched shot areas SMq andSM(q+2), are defined as reference measurement areas, and four measuringpoints 336A to 336D are set in the first reference measurement area SMq.Similarly, four measuring points 336E to 336H are set in the secondreference measurement area SM(q+2) at respective positions correspondingto the measuring points 336A to 336D.

At the measuring point 336A, amounts of positional displacement in thedirections X and Y between the critical layer vernier mark 332A and themiddle layer vernier mark 334A are measured. Similarly, amounts ofpositional displacement between the corresponding vernier marks of thetwo layers at each of the other measuring points 336B to 336D and 336Eto 336H. It should be noted that other areas in FIG. 31(a) where any oneof the critical layer shot areas and any one of the middle layer shotareas are perfectly overlaid on one another such that neither of theoverlaid shot areas extends over a plurality of middle or critical layershot areas may be used as reference measurement areas in addition to theabove.

Next, one example of a method obtaining errors of the four in-shotparameters from the results of the measurement of amounts of positionaldisplacement between two corresponding vernier marks at each of themeasuring points will be explained. Here, amounts of positionaldisplacement between the two corresponding vernier marks measured at twomeasuring points 336A and 336B, which are apart from each other in thedirection X in the first reference measurement area SMq, are denoted by(Δxa,Δya) and (Δxb,Δyb), and amounts of positional displacement betweenthe two corresponding vernier marks measured at two measuring points336C and 336D, which are apart from each other in the direction Y, aredenoted by (Δxc,Δyc) and (Δxd,Δyd). In this case, an error Δrx of the Xdirection shot magnification rx is obtained from the difference betweenthe amounts of positional displacement Δxa and Δxb, and an error Δry ofthe Y direction shot magnification ry is obtained from the differencebetween the amounts of positional displacement Δyc and Δyd. An error Δθof the shot rotation θ is obtained from the difference between theamounts of positional displacement Δya and Δyb. Further, an error Δw ofthe shot perpendicularity w is obtained from the difference between theamounts of positional displacement Δxc and Δxd and the shot rotationerror Δθ.

Further, mean values of in-shot parameter errors Δrx, Δry, Δθ and Δw,obtained in the other reference measurement areas, are determined, andthese mean values are stored in the storage unit in the controller 10Bof the scanning exposure apparatus 1B as correction values Δrx′, Δry′,Δθ′ and Δw′ for the in-shot parameters. In this embodiment, none of thereference measurement areas extend over two shot areas on either of thecritical and middle layers. Therefore, the in-shot parameter correctionvalues obtained as described above are accurate values which have gotrid of the effects of the stepping errors at the critical and middlelayers.

In this regard, let us consider a case where, in FIG. 32, the centralshot area SM(q+1), which extends over the two shot areas SNr andSN(r+1), is used as a reference measurement area, and measuring points337B and 337A are set in the two shot areas SNr and SN(r+1) within thereference measurement area. In this case, amounts of positionaldisplacement between the two corresponding vernier marks measured at thetwo measuring points 337B and 337A contain the middle layer steppingerror independently of each other. Therefore, even when an error of theshot magnification ry in the direction Y, for example, is calculatedfrom the sum of the amounts of positional displacement measured at thetwo points 337B and 337A, the calculated error contains the steppingerror. In other words, even if a reference measurement area extends overa plurality of shot areas of either layer, if a plurality of measuringpoints in the reference measurement area are set so that thedistribution of the measuring points does not extend over a plurality ofshot areas, the mixing of the stepping error can be prevented. In a casewhere amounts of positional displacement between the correspondingvernier marks are measured by another measuring device, and correctionvalues for the in-shot parameters are determined by another computer,for example, the operator inputs the correction values to the controller10B through an input unit or by on-line communication from thatcomputer. Thus, the third step is terminated.

In a case where exposure is carried out by the mix-and-match methodusing the fine stepper 1A and the scanning exposure apparatus 1B, shownin FIG. 29, after the above-described third step, first, a criticallayer pattern is formed on the wafer 20 by using the fine stepper 1A.Thereafter, before exposure for a middle layer pattern is carried out byusing the scanning exposure apparatus 1B, coordinate positions of amultiplicity of wafer marks in predetermined sample shots are measured,and values of six wafer EGA parameters and four in-shot parameters aredetermined from the result of the measurement. Thereafter, thecontroller 10B adds the correction values (Δrx′, Δry′, Δθ′, and Δw′),stored in the above-described third step, to the determined in-shotparameters (rx, ry, θ, and w), thereby obtaining corrected in-shotparameters Then, the controller 10B calculates the coordinate positionof each shot area of the critical layer by using the six wafer EGAparameters, calculates the exposure position for each shot area of themiddle layer on the basis of the calculated coordinate positions, andsequentially effects positioning (e.g. setting of the scanning startposition) of the middle layer shot areas on the basis of the calculatedexposure positions. Then, the scanning exposure apparatus 1B transfersan image of the reticle pattern onto each shot area by the scanningexposure method while correcting the image-formation characteristicsaccording to the corrected in-shot parameter values. In this embodiment,the corrected in-shot parameter values are accurate; therefore, theoverlay accuracy between the critical and middle layers is higher thanin the conventional exposure process.

Next, other examples of the fourth embodiment of the present inventionwill be explained with reference to FIGS. 33(a) to 34(c). In the exampleshown in FIGS. 33(a) to 33(c), a critical layer shot area SK shown inFIG. 33(a) is allotted one chip pattern, and a middle layer shot area SLshown in FIG. 33(b) is allotted a total of four identical chip patternsarranged in an array of two columns in the direction X and two rows inthe direction Y. The exposure apparatus for the critical layer is aone-shot exposure type projection exposure apparatus (stepper) having ademagnification ratio of 5:1, and the exposure apparatus for the middlelayer is a stepper having a demagnification ratio of 2.5:1.

Assuming that the width in the direction X of the critical layer shotarea SK is d, and the width in the direction Y of the shot area SK is c,the width in the direction X of the middle layer shot area SL is 2 d,and the width in the direction Y of the shot area SL is 2 c. Therefore,the shot area SL is twice as large as the shot area SK in each of thedirections X and Y. Accordingly, as shown in FIG. 33(c), in an-area 339where an array of four critical layer shot areas and one middle layershot area are overlaid on one another, an area where a shot area SK anda shot area SL are overlaid on one another without extending over aplurality of critical or middle layer shot areas, that is, each criticallayer shot area SK itself, is used as a reference measurement area.Therefore, two measuring points 340A and 340B are set in one referencemeasurement area 339 a, for example, and an amount of positionaldisplacement between the corresponding vernier marks of the two layersis measured at each of the measuring points 340A and 340B. By doing so,correction values for in-shot parameters, e.g. the shot magnification rxand the shot rotation 9, can be accurately obtained.

In the example shown in FIGS. 34(a) to 34(c), a first-layer shot area SOshown in FIG. 34(a) is allotted a total of six identical chip patternsarranged in an array of two columns in the direction X and three rows inthe direction Y, and a second-layer shot area SP shown in FIG. 34(b) isallotted three identical chip patterns arranged in the direction Y. Theexposure apparatus for the first layer comprising the shot areas SO is astepper, and the exposure apparatus for the second layer comprising theshot areas SP is a step-and-scan type projection exposure apparatus.

Assuming that the width in the direction X of the shot area SO is 2 b,and the width in the direction Y of the shot area SO is 3 a, the widthin the direction X of the shot area SP is b, and the width in thedirection Y of the shot area SP is 3 a. That is, the shot area SP is ½times as large as the shot area SO in the direction X, and the former isequal in size (1:1) to the latter in the direction Y. According, asshown in FIG. 34(c), in an area 341 where one first-layer shot area andtwo second-layer shot areas are overlaid on one another, an area where ashot area SO and a shot area SP are overlaid on one another withoutextending over a plurality of first- or second-layer shot areas, thatis, each second-layer shot area SP itself, is used as a referencemeasurement area. Therefore, two measuring points 342A and 342B are setin one reference measurement area 341 a, for example, and an amount ofpositional displacement between the corresponding vernier marks of thetwo layers at each of the measuring points 342A and 342B. By doing so, acorrection value for an in-shot parameter, e.g. the shot magnificationry, can be accurately obtained.

Although in the above-described embodiment a combination of two steppersor a combination of a stepper and a step-and-scan type projectionexposure apparatus is used, it should be noted that the combination ofexposure apparatuses is not necessarily limited to those describedabove. For example, step-and-scan type projection exposure apparatuseswhich are different from each other may be used as two exposureapparatuses having respective exposure fields of different sizes.

According to the exposure method of the fourth embodiment, none of theset reference measurement areas extend over a plurality of shot areas oneither of two layers (e.g. critical and middle layers). Therefore, nostepping error is contained in an amount of positional displacementbetween two corresponding overlay accuracy measuring marks measured atany of the measuring points in the reference measurement areas.Accordingly, the overlay accuracy between the two layers can be improvedby correcting the coordinates during alignment or the image-formationcharacteristics on the basis of the measured amounts of positionaldisplacement between the corresponding overlay accuracy measuring marks.

Further, according to the fourth embodiment, an area where any one of aplurality of first shot areas and any one of a plurality of second shotareas are overlaid on one another such that neither of the overlaid shotareas extends over beyond a part of that area (or neither of themextends over a plurality of first or second shot areas) is used as areference measurement area, and an amount of positional displacementbetween the corresponding overlay accuracy measuring marks (verniermarks) of the two layers is measured at each of measuring points set inpredetermined reference measurement areas. Accordingly, correctionvalues used in detection of the image positions of alignment marks(wafer marks) can be accurately obtained without being affected bystepping errors in the first and second shot areas. As a result, it ispossible to increase the overlay accuracy between a critical layerpattern and a middle layer pattern in a case where exposure is carriedout by the mix-and-match method with respect to a substrate where acritical layer and a middle layer are mixedly present. It is alsopossible to eliminate the effects of so-called seam errors between thefirst-layer shot areas and between the second-layer shot areas inaddition to the stepping error.

Further, it is possible to obtain a correction value for an in-shotparameter with high accuracy in a case where a correction value obtainedin the third step is a correction value for a parameter indicating apredetermined image-formation characteristic, which is calculated on thebasis of the positions of alignment mark images, and the parameterindicating the predetermined image-formation characteristic is at leastone parameter selected from the parameter group consisting of shotmagnification, shot rotation, and shot perpendicularity. Accordingly,the image-formation characteristics can be corrected with high accuracyby using the corrected in-shot parameter.

In a case where the first exposure apparatus is a one-shot exposure typeprojection exposure apparatus, and the second exposure apparatus is ascanning exposure type projection exposure apparatus, the exposuremethod according to the present invention is particularly effectivebecause in such a case the exposure fields of the two exposureapparatuses are likely to differ in size from each other. In the case ofa scanning exposure type projection exposure apparatus, the shotmagnification, shot rotation and shot perpendicularity can be readilycorrected at the time of exposure; therefore the overlay accuracybetween the two layers can be further increased by using parameterscorrected by the method according to the present invention.

It should be noted that the present invention is not necessarily limitedto the above-described first to fourth embodiments, but may adoptvarious arrangements without departing from the gist of the presentinvention.

What is claimed is:
 1. An exposure method in which mask patterns areoverlaid on one another on a photosensitive substrate, which is anobject to be exposed, by using a first exposure apparatus having a firstexposure field of a predetermined size on said photosensitive substrate,and a second exposure apparatus having a second exposure field which isM₁/N₁ times (M₁ and N₁ are integers; M₁>N₁) as large as said firstexposure field in a first direction and which is M₂/N₂ times (M₂ and N₂are integers; M₂≧N₂) as large as said first exposure field in a seconddirection which is perpendicular to said first direction, said exposuremethod comprising: a first step of sequentially transferring an image ofa first mask pattern, which has an alignment mark and a first overlayaccuracy measuring mark, onto said photosensitive substrate in the formof a two-dimensional array extending in said first and second directionsin units of said first exposure field by using said first exposureapparatus; a second step of transferring an image of a second maskpattern, which has a second overlay accuracy measuring mark, over aplurality of images of said first mask pattern which have beentransferred onto said photosensitive substrate in said first step, in atwo-dimensional array extending in said first and second directions onsaid photosensitive substrate in units of said second exposure fieldwith reference to a position of an image of said alignment mark by usingsaid second exposure apparatus; and a third step of dividing an exposurearea on said photosensitive substrate into a plurality of referencemeasurement areas in units of an area which is N₁ times as large as awidth of said second exposure field in said first direction on saidphotosensitive substrate and which is N₂ times as large as a width ofsaid second exposure field in said second direction on saidphotosensitive substrate, and measuring an amount of positionaldisplacement between images of said first and second overlay accuracymeasuring marks lying at mutually identical positions in a predeterminednumber of reference measurement areas selected from among said pluralityof reference measurement areas, thereby obtaining a correction valuewhich is used when the position of the image of said alignment marktransferred by said first exposure apparatus is detected by said secondexposure apparatus on the basis of said measured amount of positionaldisplacement; wherein an exposure position is corrected by using saidcorrection value obtained in said third step when overlay exposure is tobe carried out thereafter by using said second exposure apparatus withrespect to a surface of said photosensitive substrate exposed by saidfirst exposure apparatus.
 2. An exposure method according to claim 1,wherein, in said second step, said second exposure apparatus calculatesan exposure position on the basis of the image of said alignment markand by use of a predetermined coordinate transformation parameter, andin said third step, said second exposure apparatus obtains a correctionvalue for said coordinate transformation parameter.
 3. An exposuremethod in which mask patterns are overlaid on one another on aphotosensitive substrate by using a first exposure apparatus and asecond exposure apparatus having respective exposure fields of differentsizes, said exposure method comprising the steps of: sequentiallytransferring images of first and second mask patterns containing overlayaccuracy measuring marks onto a photosensitive substrate for evaluationsuch that the images of said first and second mask patterns are overlaidon one another by using said first and second exposure apparatus;measuring an amount of positional displacement between the overlaidimages of said overlay accuracy measuring marks at a predeterminedmeasuring point in a reference measurement area on said evaluationphotosensitive substrate in which a shot area formed in units of theexposure field of said first exposure apparatus and a shot area formedin units of the exposure field of said second exposure apparatus areoverlaid on one another such that neither of said overlaid shot areasextends beyond a part of said reference measurement area; and effectingalignment or correction of image-formation characteristics on the basisof a result of said measurement when exposure is to be carried out bysaid second exposure apparatus with respect to a surface of saidphotosensitive substrate exposed by said first exposure apparatus.
 4. Anexposure method in which mask patterns are overlaid on one another on aphotosensitive substrate, which is an object to be exposed, by using afirst exposure apparatus having a first exposure field of apredetermined size on said photosensitive substrate, and a secondexposure apparatus having a second exposure field which is M₁/N₁ times(M₁ and N₁ are integers; M₁≈N₁) as large as said first exposure field ina first direction and which is M₂/N₂ times (M₂ and N₂ are integers) aslarge as said first exposure field in a second direction which isperpendicular to said first direction, said exposure method comprising:a first step of sequentially transferring an image of a first maskpattern, which has an alignment mark and a first overlay accuracymeasuring mark, onto a plurality of first shot areas arrayed on saidphotosensitive substrate in units of said first exposure field by usingsaid first exposure apparatus; a second step of sequentiallytransferring an image of a second mask pattern, which has a secondoverlay accuracy measuring mark, onto a plurality of second shot areasarrayed on said photosensitive substrate, exposed in said first step, inunits of said second exposure field with reference to a position of animage of said alignment mark by using said second exposure apparatus;and a third step of defining a plurality of reference measurement areason said photosensitive substrate in each of which any one of said firstshot areas and any one of said second shot areas are overlaid on oneanother such that neither of said overlaid shot areas extends beyond apart of said reference measurement area, and measuring an amount ofpositional displacement between images of said first and second overlayaccuracy measuring marks lying at mutually identical positions in apredetermined number of reference measurement areas selected from amongsaid plurality of reference measurement areas, thereby obtaining acorrection value which is used when the position of the image of saidalignment mark transferred by said first exposure apparatus is detectedby said second exposure apparatus on the basis of said measured amountof positional displacement.
 5. An exposure method according to claim 4,wherein the correction value obtained in said third step is a correctionvalue for a parameter indicating a predetermined image-formationcharacteristic calculated on the basis of the position of the image ofsaid alignment mark, said parameter being at least one parameterselected from a parameter group consisting of shot magnification, shotrotation, and shot perpendicularity, and wherein said image-formationcharacteristic is corrected by using the correction value obtained insaid third step when overlay exposure is to be carried out thereafter byusing said second exposure apparatus with respect to a surface of saidphotosensitive substrate exposed by said first exposure apparatus.
 6. Anexposure method according to claim 4, wherein said first exposureapparatus is a one-shot exposure type projection exposure apparatus, andsaid second exposure apparatus is a scanning exposure type projectionexposure apparatus.
 7. An exposure method in which a substrate isexposed with a second pattern by using a second exposure apparatus, themethod comprising: providing the substrate on which a plurality of firstshot areas are formed, the plurality of first shot areas on thesubstrate being formed by exposing the substrate with a first pattern byusing a first exposure apparatus before exposing the substrate with thesecond pattern, each of the first shot areas having M partial areasalong a first direction (M is an integer), and the first and secondexposure apparatus having respective exposure fields of different sizes;and performing a scanning exposure in which each of a plurality ofsecond shot areas is exposed while moving the substrate along the firstdirection in the second exposure apparatus, each of the second shotareas having N partial areas along the first direction (N is an integer;N≈M), each partial area of the first shot areas substantiallyoverlapping with each partial area of the second shot areas.
 8. Anexposure method according to claim 7, wherein the substrate is exposedby using the first exposure apparatus so that the first shot areasformed into a plurality of rows parallel to the first direction and thefirst shot areas adjacent to each other in the first direction arearranged without positional deviation in a second directionperpendicular to the first direction.
 9. An exposure method according toclaim 7, wherein a position of the substrate in the second directionduring scanning exposure for one second shot area SC1 is different froma position of the substrate in the second direction during scanningexposure for another second shot area SC2 which is adjacent to the onesecond shot area SCI in the first direction.
 10. An exposure methodaccording to claim 9, further comprising; obtaining shot rotationinformation θ of each of the plurality of first shot areas, wherein thesubstrate is moved in the first direction based on the obtained shotrotation information θ.
 11. An exposure method according to claim 7,further comprising: obtaining shot rotation information θ of each of theplurality of first shot areas, wherein the substrate is moved in thefirst direction based on the obtained shot rotation information θ. 12.An exposure method according to claim 7, wherein M is an even number andN is an odd number.